How does Linux move from an awake machine to a hibernating one?
How does it then manage to restore all state?
These questions led me to read way too much C in trying to figure out
how this particular hardware/software boundary is navigated.
This investigation will be split into a few parts, with the first one going
from invocation of hibernation to synchronizing all filesystems to disk.
This article has been written using Linux version 6.9.9,
the source of which can be found in many places, but can be navigated
easily through the Bootlin Elixir Cross-Referencer:
https://elixir.bootlin.com/linux/v6.9.9/source
Each code snippet will begin with a link to the above giving
the file path and the line number of the beginning of the snippet.
A Starting Point for Investigation: /sys/power/state and /sys/power/disk
These two system files exist to allow debugging of hibernation,
and thus control the exact state used directly.
Writing specific values to the state file controls the exact sleep mode used
and disk controls the specific hibernation mode.
This is extremely handy as an entry point to understand how these systems work,
since we can just follow what happens when they are written to.
Show and Store Functions
These two files are defined using the power_attr macro:
kernel/power/power.h:80
#define power_attr(_name) \
static struct kobj_attribute _name##_attr = { \
.attr = { \
.name = __stringify(_name), \
.mode = 0644, \
}, \
.show = _name##_show, \
.store = _name##_store, \
}
show is called on reads and store on writes.
state_show is a little boring for our purposes, as it just prints all the
available sleep states.
kernel/power/main.c:657
/*
* state - control system sleep states.
*
* show() returns available sleep state labels, which may be "mem", "standby",
* "freeze" and "disk" (hibernation).
* See Documentation/admin-guide/pm/sleep-states.rst for a description of
* what they mean.
*
* store() accepts one of those strings, translates it into the proper
* enumerated value, and initiates a suspend transition.
*/
static ssize_t state_show(struct kobject *kobj, struct kobj_attribute *attr,
char *buf)
{
char *s = buf;
#ifdef CONFIG_SUSPEND
suspend_state_t i;
for (i = PM_SUSPEND_MIN; i < PM_SUSPEND_MAX; i++)
if (pm_states[i])
s += sprintf(s,"%s ", pm_states[i]);
#endif
if (hibernation_available())
s += sprintf(s, "disk ");
if (s != buf)
/* convert the last space to a newline */
*(s-1) = '\n';
return (s - buf);
}
state_store, however, provides our entry point.
If the string “disk” is written to the state file, it calls hibernate().
This is our entry point.
kernel/power/main.c:715
static ssize_t state_store(struct kobject *kobj, struct kobj_attribute *attr,
const char *buf, size_t n)
{
suspend_state_t state;
int error;
error = pm_autosleep_lock();
if (error)
return error;
if (pm_autosleep_state() > PM_SUSPEND_ON) {
error = -EBUSY;
goto out;
}
state = decode_state(buf, n);
if (state < PM_SUSPEND_MAX) {
if (state == PM_SUSPEND_MEM)
state = mem_sleep_current;
error = pm_suspend(state);
} else if (state == PM_SUSPEND_MAX) {
error = hibernate();
} else {
error = -EINVAL;
}
out:
pm_autosleep_unlock();
return error ? error : n;
}
kernel/power/main.c:688
static suspend_state_t decode_state(const char *buf, size_t n)
{
#ifdef CONFIG_SUSPEND
suspend_state_t state;
#endif
char *p;
int len;
p = memchr(buf, '\n', n);
len = p ? p - buf : n;
/* Check hibernation first. */
if (len == 4 && str_has_prefix(buf, "disk"))
return PM_SUSPEND_MAX;
#ifdef CONFIG_SUSPEND
for (state = PM_SUSPEND_MIN; state < PM_SUSPEND_MAX; state++) {
const char *label = pm_states[state];
if (label && len == strlen(label) && !strncmp(buf, label, len))
return state;
}
#endif
return PM_SUSPEND_ON;
}
Could we have figured this out just via function names?
Sure, but this way we know for sure that nothing else is happening before this
function is called.
Autosleep
Our first detour is into the autosleep system.
When checking the state above, you may notice that
the kernel grabs the pm_autosleep_lock before checking the current
state.
autosleep is a mechanism originally from Android
that sends the entire system to either suspend or hibernate whenever it is
not actively working on anything.
This is not enabled for most desktop configurations, since it’s primarily
for mobile systems and inverts the standard suspend and hibernate interactions.
This system is implemented as a workqueue
that checks the current number of wakeup events, processes and drivers that
need to run, and if there aren’t any, then the system is put
into the autosleep state, typically suspend. However, it could be hibernate if
configured that way via /sys/power/autosleep in a similar manner to
using /sys/power/state to manually enable hibernation.
kernel/power/main.c:841
static ssize_t autosleep_store(struct kobject *kobj,
struct kobj_attribute *attr,
const char *buf, size_t n)
{
suspend_state_t state = decode_state(buf, n);
int error;
if (state == PM_SUSPEND_ON
&& strcmp(buf, "off") && strcmp(buf, "off\n"))
return -EINVAL;
if (state == PM_SUSPEND_MEM)
state = mem_sleep_current;
error = pm_autosleep_set_state(state);
return error ? error : n;
}
power_attr(autosleep);
#endif /* CONFIG_PM_AUTOSLEEP */
kernel/power/autosleep.c:24
static DEFINE_MUTEX(autosleep_lock);
static struct wakeup_source *autosleep_ws;
static void try_to_suspend(struct work_struct *work)
{
unsigned int initial_count, final_count;
if (!pm_get_wakeup_count(&initial_count, true))
goto out;
mutex_lock(&autosleep_lock);
if (!pm_save_wakeup_count(initial_count) ||
system_state != SYSTEM_RUNNING) {
mutex_unlock(&autosleep_lock);
goto out;
}
if (autosleep_state == PM_SUSPEND_ON) {
mutex_unlock(&autosleep_lock);
return;
}
if (autosleep_state >= PM_SUSPEND_MAX)
hibernate();
else
pm_suspend(autosleep_state);
mutex_unlock(&autosleep_lock);
if (!pm_get_wakeup_count(&final_count, false))
goto out;
/*
* If the wakeup occurred for an unknown reason, wait to prevent the
* system from trying to suspend and waking up in a tight loop.
*/
if (final_count == initial_count)
schedule_timeout_uninterruptible(HZ / 2);
out:
queue_up_suspend_work();
}
static DECLARE_WORK(suspend_work, try_to_suspend);
void queue_up_suspend_work(void)
{
if (autosleep_state > PM_SUSPEND_ON)
queue_work(autosleep_wq, &suspend_work);
}
The Steps of Hibernation
Hibernation Kernel Config
It’s important to note that most of the hibernate-specific functions below
do nothing unless you’ve defined CONFIG_HIBERNATION in your Kconfig.
As an example, hibernate itself is defined as the following if
CONFIG_HIBERNATE is not set.
include/linux/suspend.h:407
static inline int hibernate(void) { return -ENOSYS; }
Check if Hibernation is Available
We begin by confirming that we actually can perform hibernation,
via the hibernation_available function.
kernel/power/hibernate.c:742
if (!hibernation_available()) {
pm_pr_dbg("Hibernation not available.\n");
return -EPERM;
}
kernel/power/hibernate.c:92
bool hibernation_available(void)
{
return nohibernate == 0 &&
!security_locked_down(LOCKDOWN_HIBERNATION) &&
!secretmem_active() && !cxl_mem_active();
}
nohibernate is controlled by the kernel command line, it’s set via
either nohibernate or hibernate=no.
security_locked_down is a hook for Linux Security Modules to prevent
hibernation. This is used to prevent hibernating to an unencrypted storage
device, as specified in the manual page
kernel_lockdown(7).
Interestingly, either level of lockdown, integrity or confidentiality,
locks down hibernation because with the ability to hibernate you can extract
bascially anything from memory and even reboot into a modified kernel image.
secretmem_active checks whether there is any active use of
memfd_secret, and if so it prevents hibernation.
memfd_secret returns a file descriptor that can be mapped into a process
but is specifically unmapped from the kernel’s memory space.
Hibernating with memory that not even the kernel is supposed to
access would expose that memory to whoever could access the hibernation image.
This particular feature of secret memory was apparently
controversial, though not as
controversial as performance concerns around fragmentation when unmapping
kernel memory
(which did not end up being a real problem).
cxl_mem_active just checks whether any CXL memory is active.
A full explanation is provided in the
commit introducing this check
but there’s also a shortened explanation from cxl_mem_probe that
sets the relevant flag when initializing a CXL memory device.
drivers/cxl/mem.c:186
* The kernel may be operating out of CXL memory on this device,
* there is no spec defined way to determine whether this device
* preserves contents over suspend, and there is no simple way
* to arrange for the suspend image to avoid CXL memory which
* would setup a circular dependency between PCI resume and save
* state restoration.
Check Compression
The next check is for whether compression support is enabled, and if so
whether the requested algorithm is enabled.
kernel/power/hibernate.c:747
/*
* Query for the compression algorithm support if compression is enabled.
*/
if (!nocompress) {
strscpy(hib_comp_algo, hibernate_compressor, sizeof(hib_comp_algo));
if (crypto_has_comp(hib_comp_algo, 0, 0) != 1) {
pr_err("%s compression is not available\n", hib_comp_algo);
return -EOPNOTSUPP;
}
}
The nocompress flag is set via the hibernate command line parameter,
setting hibernate=nocompress.
If compression is enabled, then hibernate_compressor is copied to
hib_comp_algo. This synchronizes the current requested compression
setting (hibernate_compressor) with the current compression setting
(hib_comp_algo).
Both values are character arrays of size CRYPTO_MAX_ALG_NAME
(128 in this kernel).
kernel/power/hibernate.c:50
static char hibernate_compressor[CRYPTO_MAX_ALG_NAME] = CONFIG_HIBERNATION_DEF_COMP;
/*
* Compression/decompression algorithm to be used while saving/loading
* image to/from disk. This would later be used in 'kernel/power/swap.c'
* to allocate comp streams.
*/
char hib_comp_algo[CRYPTO_MAX_ALG_NAME];
hibernate_compressor defaults to lzo if that algorithm is enabled, otherwise to lz4 if
enabled. It can be overwritten using the hibernate.compressor setting to
either lzo or lz4.
kernel/power/Kconfig:95
choice
prompt "Default compressor"
default HIBERNATION_COMP_LZO
depends on HIBERNATION
config HIBERNATION_COMP_LZO
bool "lzo"
depends on CRYPTO_LZO
config HIBERNATION_COMP_LZ4
bool "lz4"
depends on CRYPTO_LZ4
endchoice
config HIBERNATION_DEF_COMP
string
default "lzo" if HIBERNATION_COMP_LZO
default "lz4" if HIBERNATION_COMP_LZ4
help
Default compressor to be used for hibernation.
kernel/power/hibernate.c:1425
static const char * const comp_alg_enabled[] = {
#if IS_ENABLED(CONFIG_CRYPTO_LZO)
COMPRESSION_ALGO_LZO,
#endif
#if IS_ENABLED(CONFIG_CRYPTO_LZ4)
COMPRESSION_ALGO_LZ4,
#endif
};
static int hibernate_compressor_param_set(const char *compressor,
const struct kernel_param *kp)
{
unsigned int sleep_flags;
int index, ret;
sleep_flags = lock_system_sleep();
index = sysfs_match_string(comp_alg_enabled, compressor);
if (index >= 0) {
ret = param_set_copystring(comp_alg_enabled[index], kp);
if (!ret)
strscpy(hib_comp_algo, comp_alg_enabled[index],
sizeof(hib_comp_algo));
} else {
ret = index;
}
unlock_system_sleep(sleep_flags);
if (ret)
pr_debug("Cannot set specified compressor %s\n",
compressor);
return ret;
}
static const struct kernel_param_ops hibernate_compressor_param_ops = {
.set = hibernate_compressor_param_set,
.get = param_get_string,
};
static struct kparam_string hibernate_compressor_param_string = {
.maxlen = sizeof(hibernate_compressor),
.string = hibernate_compressor,
};
We then check whether the requested algorithm is supported via crypto_has_comp.
If not, we bail out of the whole operation with EOPNOTSUPP.
As part of crypto_has_comp we perform any needed initialization of the
algorithm, loading kernel modules and running initialization code as needed.
Grab Locks
The next step is to grab the sleep and hibernation locks via
lock_system_sleep and hibernate_acquire.
kernel/power/hibernate.c:758
sleep_flags = lock_system_sleep();
/* The snapshot device should not be opened while we're running */
if (!hibernate_acquire()) {
error = -EBUSY;
goto Unlock;
}
First, lock_system_sleep marks the current thread as not freezable, which
will be important later. It then grabs the system_transistion_mutex,
which locks taking snapshots or modifying how they are taken,
resuming from a hibernation image, entering any suspend state, or rebooting.
The GFP Mask
The kernel also issues a warning if the gfp mask is changed via either
pm_restore_gfp_mask or pm_restrict_gfp_mask
without holding the system_transistion_mutex.
GFP flags tell the kernel how it is permitted to handle a request for memory.
include/linux/gfp_types.h:12
* GFP flags are commonly used throughout Linux to indicate how memory
* should be allocated. The GFP acronym stands for get_free_pages(),
* the underlying memory allocation function. Not every GFP flag is
* supported by every function which may allocate memory.
In the case of hibernation specifically we care about the IO and FS flags,
which are reclaim operators, ways the system is permitted to attempt to free
up memory in order to satisfy a specific request for memory.
include/linux/gfp_types.h:176
* Reclaim modifiers
* -----------------
* Please note that all the following flags are only applicable to sleepable
* allocations (e.g. %GFP_NOWAIT and %GFP_ATOMIC will ignore them).
*
* %__GFP_IO can start physical IO.
*
* %__GFP_FS can call down to the low-level FS. Clearing the flag avoids the
* allocator recursing into the filesystem which might already be holding
* locks.
gfp_allowed_mask sets which flags are permitted to be set at the current time.
As the comment below outlines, preventing these flags from being set
avoids situations where the kernel needs to do I/O to allocate memory
(e.g. read/writing swap) but the
devices it needs to read/write to/from are not currently available.
kernel/power/main.c:24
/*
* The following functions are used by the suspend/hibernate code to temporarily
* change gfp_allowed_mask in order to avoid using I/O during memory allocations
* while devices are suspended. To avoid races with the suspend/hibernate code,
* they should always be called with system_transition_mutex held
* (gfp_allowed_mask also should only be modified with system_transition_mutex
* held, unless the suspend/hibernate code is guaranteed not to run in parallel
* with that modification).
*/
static gfp_t saved_gfp_mask;
void pm_restore_gfp_mask(void)
{
WARN_ON(!mutex_is_locked(&system_transition_mutex));
if (saved_gfp_mask) {
gfp_allowed_mask = saved_gfp_mask;
saved_gfp_mask = 0;
}
}
void pm_restrict_gfp_mask(void)
{
WARN_ON(!mutex_is_locked(&system_transition_mutex));
WARN_ON(saved_gfp_mask);
saved_gfp_mask = gfp_allowed_mask;
gfp_allowed_mask &= ~(__GFP_IO | __GFP_FS);
}
Sleep Flags
After grabbing the system_transition_mutex the kernel then returns and
captures the previous state of the threads flags in sleep_flags.
This is used later to remove PF_NOFREEZE if it wasn’t previously set on the
current thread.
kernel/power/main.c:52
unsigned int lock_system_sleep(void)
{
unsigned int flags = current->flags;
current->flags |= PF_NOFREEZE;
mutex_lock(&system_transition_mutex);
return flags;
}
EXPORT_SYMBOL_GPL(lock_system_sleep);
include/linux/sched.h:1633
#define PF_NOFREEZE 0x00008000 /* This thread should not be frozen */
Then we grab the hibernate-specific semaphore to ensure no one can open a
snapshot or resume from it while we perform hibernation.
Additionally this lock is used to prevent hibernate_quiet_exec,
which is used by the nvdimm driver to active its firmware with all
processes and devices frozen, ensuring it is the only thing running at that
time.
kernel/power/hibernate.c:82
bool hibernate_acquire(void)
{
return atomic_add_unless(&hibernate_atomic, -1, 0);
}
Prepare Console
The kernel next calls pm_prepare_console.
This function only does anything if CONFIG_VT_CONSOLE_SLEEP has been set.
This prepares the virtual terminal for a suspend state, switching away to
a console used only for the suspend state if needed.
kernel/power/console.c:130
void pm_prepare_console(void)
{
if (!pm_vt_switch())
return;
orig_fgconsole = vt_move_to_console(SUSPEND_CONSOLE, 1);
if (orig_fgconsole < 0)
return;
orig_kmsg = vt_kmsg_redirect(SUSPEND_CONSOLE);
return;
}
The first thing is to check whether we actually need to switch the VT
kernel/power/console.c:94
/*
* There are three cases when a VT switch on suspend/resume are required:
* 1) no driver has indicated a requirement one way or another, so preserve
* the old behavior
* 2) console suspend is disabled, we want to see debug messages across
* suspend/resume
* 3) any registered driver indicates it needs a VT switch
*
* If none of these conditions is present, meaning we have at least one driver
* that doesn't need the switch, and none that do, we can avoid it to make
* resume look a little prettier (and suspend too, but that's usually hidden,
* e.g. when closing the lid on a laptop).
*/
static bool pm_vt_switch(void)
{
struct pm_vt_switch *entry;
bool ret = true;
mutex_lock(&vt_switch_mutex);
if (list_empty(&pm_vt_switch_list))
goto out;
if (!console_suspend_enabled)
goto out;
list_for_each_entry(entry, &pm_vt_switch_list, head) {
if (entry->required)
goto out;
}
ret = false;
out:
mutex_unlock(&vt_switch_mutex);
return ret;
}
There is an explanation of the conditions under which a switch is performed
in the comment above the function, but we’ll also walk through the steps here.
Firstly we grab the vt_switch_mutex to ensure nothing will modify the list
while we’re looking at it.
We then examine the pm_vt_switch_list.
This list is used to indicate the drivers that require a switch during suspend.
They register this requirement, or the lack thereof, via
pm_vt_switch_required.
kernel/power/console.c:31
/**
* pm_vt_switch_required - indicate VT switch at suspend requirements
* @dev: device
* @required: if true, caller needs VT switch at suspend/resume time
*
* The different console drivers may or may not require VT switches across
* suspend/resume, depending on how they handle restoring video state and
* what may be running.
*
* Drivers can indicate support for switchless suspend/resume, which can
* save time and flicker, by using this routine and passing 'false' as
* the argument. If any loaded driver needs VT switching, or the
* no_console_suspend argument has been passed on the command line, VT
* switches will occur.
*/
void pm_vt_switch_required(struct device *dev, bool required)
Next, we check console_suspend_enabled. This is set to false
by the kernel parameter no_console_suspend, but defaults to true.
Finally, if there are any entries in the pm_vt_switch_list, then we
check to see if any of them require a VT switch.
Only if none of these conditions apply, then we return false.
If a VT switch is in fact required, then we move first the currently active
virtual terminal/console (vt_move_to_console)
and then the current location of kernel messages (vt_kmsg_redirect)
to the SUSPEND_CONSOLE.
The SUSPEND_CONSOLE is the last entry in the list of possible
consoles, and appears to just be a black hole to throw away messages.
kernel/power/console.c:16
#define SUSPEND_CONSOLE (MAX_NR_CONSOLES-1)
Interestingly, these are separate functions because you can use
TIOCL_SETKMSGREDIRECT (an ioctl) to send kernel messages to a specific virtual terminal,
but by default its the same as the currently active console.
The locations of the previously active console and the previous kernel
messages location are stored in orig_fgconsole and orig_kmsg, to
restore the state of the console and kernel messages after the machine wakes
up again.
Interestingly, this means orig_fgconsole also ends up storing any errors,
so has to be checked to ensure it’s not less than zero before we try to do
anything with the kernel messages on both suspend and resume.
drivers/tty/vt/vt_ioctl.c:1268
/* Perform a kernel triggered VT switch for suspend/resume */
static int disable_vt_switch;
int vt_move_to_console(unsigned int vt, int alloc)
{
int prev;
console_lock();
/* Graphics mode - up to X */
if (disable_vt_switch) {
console_unlock();
return 0;
}
prev = fg_console;
if (alloc && vc_allocate(vt)) {
/* we can't have a free VC for now. Too bad,
* we don't want to mess the screen for now. */
console_unlock();
return -ENOSPC;
}
if (set_console(vt)) {
/*
* We're unable to switch to the SUSPEND_CONSOLE.
* Let the calling function know so it can decide
* what to do.
*/
console_unlock();
return -EIO;
}
console_unlock();
if (vt_waitactive(vt + 1)) {
pr_debug("Suspend: Can't switch VCs.");
return -EINTR;
}
return prev;
}
Unlike most other locking functions we’ve seen so far, console_lock
needs to be careful to ensure nothing else is panicking and needs to
dump to the console before grabbing the semaphore for the console
and setting a couple flags.
Panics
Panics are tracked via an atomic integer set to the id of the processor
currently panicking.
kernel/printk/printk.c:2649
/**
* console_lock - block the console subsystem from printing
*
* Acquires a lock which guarantees that no consoles will
* be in or enter their write() callback.
*
* Can sleep, returns nothing.
*/
void console_lock(void)
{
might_sleep();
/* On panic, the console_lock must be left to the panic cpu. */
while (other_cpu_in_panic())
msleep(1000);
down_console_sem();
console_locked = 1;
console_may_schedule = 1;
}
EXPORT_SYMBOL(console_lock);
kernel/printk/printk.c:362
/*
* Return true if a panic is in progress on a remote CPU.
*
* On true, the local CPU should immediately release any printing resources
* that may be needed by the panic CPU.
*/
bool other_cpu_in_panic(void)
{
return (panic_in_progress() && !this_cpu_in_panic());
}
kernel/printk/printk.c:345
static bool panic_in_progress(void)
{
return unlikely(atomic_read(&panic_cpu) != PANIC_CPU_INVALID);
}
kernel/printk/printk.c:350
/* Return true if a panic is in progress on the current CPU. */
bool this_cpu_in_panic(void)
{
/*
* We can use raw_smp_processor_id() here because it is impossible for
* the task to be migrated to the panic_cpu, or away from it. If
* panic_cpu has already been set, and we're not currently executing on
* that CPU, then we never will be.
*/
return unlikely(atomic_read(&panic_cpu) == raw_smp_processor_id());
}
console_locked is a debug value, used to indicate that the lock should be
held, and our first indication that this whole virtual terminal system is
more complex than might initially be expected.
kernel/printk/printk.c:373
/*
* This is used for debugging the mess that is the VT code by
* keeping track if we have the console semaphore held. It's
* definitely not the perfect debug tool (we don't know if _WE_
* hold it and are racing, but it helps tracking those weird code
* paths in the console code where we end up in places I want
* locked without the console semaphore held).
*/
static int console_locked;
console_may_schedule is used to see if we are permitted to sleep
and schedule other work while we hold this lock.
As we’ll see later, the virtual terminal subsystem is not re-entrant,
so there’s all sorts of hacks in here to ensure we don’t leave important
code sections that can’t be safely resumed.
Disable VT Switch
As the comment below lays out, when another program is handling graphical
display anyway, there’s no need to do any of this, so the kernel provides
a switch to turn the whole thing off.
Interestingly, this appears to only be used by three drivers,
so the specific hardware support required must not be particularly common.
drivers/gpu/drm/omapdrm/dss
drivers/video/fbdev/geode
drivers/video/fbdev/omap2
drivers/tty/vt/vt_ioctl.c:1308
/*
* Normally during a suspend, we allocate a new console and switch to it.
* When we resume, we switch back to the original console. This switch
* can be slow, so on systems where the framebuffer can handle restoration
* of video registers anyways, there's little point in doing the console
* switch. This function allows you to disable it by passing it '0'.
*/
void pm_set_vt_switch(int do_switch)
{
console_lock();
disable_vt_switch = !do_switch;
console_unlock();
}
EXPORT_SYMBOL(pm_set_vt_switch);
The rest of the vt_switch_console function is pretty normal,
however, simply allocating space if needed to create the requested
virtual terminal
and then setting the current virtual terminal via set_console.
Virtual Terminal Set Console
With set_console, we begin (as if we haven’t been already) to enter the
madness that is the virtual terminal subsystem.
As mentioned previously, modifications to its state must be made very
carefully, as other stuff happening at the same time could create complete
messes.
All this to say, calling set_console does not actually perform any
work to change the state of the current console.
Instead it indicates what changes it wants and then schedules that work.
drivers/tty/vt/vt.c:3153
int set_console(int nr)
{
struct vc_data *vc = vc_cons[fg_console].d;
if (!vc_cons_allocated(nr) || vt_dont_switch ||
(vc->vt_mode.mode == VT_AUTO && vc->vc_mode == KD_GRAPHICS)) {
/*
* Console switch will fail in console_callback() or
* change_console() so there is no point scheduling
* the callback
*
* Existing set_console() users don't check the return
* value so this shouldn't break anything
*/
return -EINVAL;
}
want_console = nr;
schedule_console_callback();
return 0;
}
The check for vc->vc_mode == KD_GRAPHICS is where most end-user graphical
desktops will bail out of this change, as they’re in graphics mode and don’t
need to switch away to the suspend console.
vt_dont_switch is a flag used by the ioctls VT_LOCKSWITCH and
VT_UNLOCKSWITCH to prevent the system from switching virtual terminal
devices when the user has explicitly locked it.
VT_AUTO is a flag indicating that automatic virtual terminal switching is enabled,
and thus deliberate switching to a suspend terminal is not required.
However, if you do run your machine from a virtual terminal, then we
indicate to the system that we want to change to the requested virtual terminal
via the want_console variable
and schedule a callback via schedule_console_callback.
drivers/tty/vt/vt.c:315
void schedule_console_callback(void)
{
schedule_work(&console_work);
}
console_work is a workqueue that will execute the given task asynchronously.
Console Callback
drivers/tty/vt/vt.c:3109
/*
* This is the console switching callback.
*
* Doing console switching in a process context allows
* us to do the switches asynchronously (needed when we want
* to switch due to a keyboard interrupt). Synchronization
* with other console code and prevention of re-entrancy is
* ensured with console_lock.
*/
static void console_callback(struct work_struct *ignored)
{
console_lock();
if (want_console >= 0) {
if (want_console != fg_console &&
vc_cons_allocated(want_console)) {
hide_cursor(vc_cons[fg_console].d);
change_console(vc_cons[want_console].d);
/* we only changed when the console had already
been allocated - a new console is not created
in an interrupt routine */
}
want_console = -1;
}
...
console_callback first looks to see if there is a console change wanted
via want_console and then changes to it if it’s not the current console and
has been allocated already.
We do first remove any cursor state with hide_cursor.
drivers/tty/vt/vt.c:841
static void hide_cursor(struct vc_data *vc)
{
if (vc_is_sel(vc))
clear_selection();
vc->vc_sw->con_cursor(vc, false);
hide_softcursor(vc);
}
A full dive into the tty driver is a task for another time, but this
should give a general sense of how this system interacts with hibernation.
Notify Power Management Call Chain
kernel/power/hibernate.c:767
pm_notifier_call_chain_robust(PM_HIBERNATION_PREPARE, PM_POST_HIBERNATION)
This will call a chain of power management callbacks, passing first
PM_HIBERNATION_PREPARE and then PM_POST_HIBERNATION on startup or
on error with another callback.
kernel/power/main.c:98
int pm_notifier_call_chain_robust(unsigned long val_up, unsigned long val_down)
{
int ret;
ret = blocking_notifier_call_chain_robust(&pm_chain_head, val_up, val_down, NULL);
return notifier_to_errno(ret);
}
The power management notifier is a blocking notifier chain, which means it
has the following properties.
include/linux/notifier.h:23
* Blocking notifier chains: Chain callbacks run in process context.
* Callouts are allowed to block.
The callback chain is a linked list with each entry containing a priority
and a function to call. The function technically takes in a data value,
but it is always NULL for the power management chain.
include/linux/notifier.h:49
struct notifier_block;
typedef int (*notifier_fn_t)(struct notifier_block *nb,
unsigned long action, void *data);
struct notifier_block {
notifier_fn_t notifier_call;
struct notifier_block __rcu *next;
int priority;
};
The head of the linked list is protected by a read-write semaphore.
include/linux/notifier.h:65
struct blocking_notifier_head {
struct rw_semaphore rwsem;
struct notifier_block __rcu *head;
};
Because it is prioritized, appending to the list requires walking it until
an item with lower priority is found to insert the current item before.
kernel/notifier.c:252
/*
* Blocking notifier chain routines. All access to the chain is
* synchronized by an rwsem.
*/
static int __blocking_notifier_chain_register(struct blocking_notifier_head *nh,
struct notifier_block *n,
bool unique_priority)
{
int ret;
/*
* This code gets used during boot-up, when task switching is
* not yet working and interrupts must remain disabled. At
* such times we must not call down_write().
*/
if (unlikely(system_state == SYSTEM_BOOTING))
return notifier_chain_register(&nh->head, n, unique_priority);
down_write(&nh->rwsem);
ret = notifier_chain_register(&nh->head, n, unique_priority);
up_write(&nh->rwsem);
return ret;
}
kernel/notifier.c:20
/*
* Notifier chain core routines. The exported routines below
* are layered on top of these, with appropriate locking added.
*/
static int notifier_chain_register(struct notifier_block **nl,
struct notifier_block *n,
bool unique_priority)
{
while ((*nl) != NULL) {
if (unlikely((*nl) == n)) {
WARN(1, "notifier callback %ps already registered",
n->notifier_call);
return -EEXIST;
}
if (n->priority > (*nl)->priority)
break;
if (n->priority == (*nl)->priority && unique_priority)
return -EBUSY;
nl = &((*nl)->next);
}
n->next = *nl;
rcu_assign_pointer(*nl, n);
trace_notifier_register((void *)n->notifier_call);
return 0;
}
Each callback can return one of a series of options.
include/linux/notifier.h:18
#define NOTIFY_DONE 0x0000 /* Don't care */
#define NOTIFY_OK 0x0001 /* Suits me */
#define NOTIFY_STOP_MASK 0x8000 /* Don't call further */
#define NOTIFY_BAD (NOTIFY_STOP_MASK|0x0002)
/* Bad/Veto action */
When notifying the chain, if a function returns STOP or BAD then
the previous parts of the chain are called again with PM_POST_HIBERNATION
and an error is returned.
kernel/notifier.c:107
/**
* notifier_call_chain_robust - Inform the registered notifiers about an event
* and rollback on error.
* @nl: Pointer to head of the blocking notifier chain
* @val_up: Value passed unmodified to the notifier function
* @val_down: Value passed unmodified to the notifier function when recovering
* from an error on @val_up
* @v: Pointer passed unmodified to the notifier function
*
* NOTE: It is important the @nl chain doesn't change between the two
* invocations of notifier_call_chain() such that we visit the
* exact same notifier callbacks; this rules out any RCU usage.
*
* Return: the return value of the @val_up call.
*/
static int notifier_call_chain_robust(struct notifier_block **nl,
unsigned long val_up, unsigned long val_down,
void *v)
{
int ret, nr = 0;
ret = notifier_call_chain(nl, val_up, v, -1, &nr);
if (ret & NOTIFY_STOP_MASK)
notifier_call_chain(nl, val_down, v, nr-1, NULL);
return ret;
}
Each of these callbacks tends to be quite driver-specific, so we’ll cease
discussion of this here.
Sync Filesystems
The next step is to ensure all filesystems have been synchronized to disk.
This is performed via a simple helper function that times how long the full
synchronize operation, ksys_sync takes.
kernel/power/main.c:69
void ksys_sync_helper(void)
{
ktime_t start;
long elapsed_msecs;
start = ktime_get();
ksys_sync();
elapsed_msecs = ktime_to_ms(ktime_sub(ktime_get(), start));
pr_info("Filesystems sync: %ld.%03ld seconds\n",
elapsed_msecs / MSEC_PER_SEC, elapsed_msecs % MSEC_PER_SEC);
}
EXPORT_SYMBOL_GPL(ksys_sync_helper);
ksys_sync wakes and instructs a set of flusher threads to write out
every filesystem, first their inodes, then the full filesystem, and
then finally all block devices, to ensure all pages are written out
to disk.
fs/sync.c:87
/*
* Sync everything. We start by waking flusher threads so that most of
* writeback runs on all devices in parallel. Then we sync all inodes reliably
* which effectively also waits for all flusher threads to finish doing
* writeback. At this point all data is on disk so metadata should be stable
* and we tell filesystems to sync their metadata via ->sync_fs() calls.
* Finally, we writeout all block devices because some filesystems (e.g. ext2)
* just write metadata (such as inodes or bitmaps) to block device page cache
* and do not sync it on their own in ->sync_fs().
*/
void ksys_sync(void)
{
int nowait = 0, wait = 1;
wakeup_flusher_threads(WB_REASON_SYNC);
iterate_supers(sync_inodes_one_sb, NULL);
iterate_supers(sync_fs_one_sb, &nowait);
iterate_supers(sync_fs_one_sb, &wait);
sync_bdevs(false);
sync_bdevs(true);
if (unlikely(laptop_mode))
laptop_sync_completion();
}
It follows an interesting pattern of using iterate_supers to run both
sync_inodes_one_sb and then sync_fs_one_sb on each known filesystem.
It also calls both sync_fs_one_sb and sync_bdevs twice, first without waiting
for any operations to complete and then again waiting for completion.
When laptop_mode is enabled the system runs additional filesystem synchronization
operations after the specified delay without any writes.
mm/page-writeback.c:111
/*
* Flag that puts the machine in "laptop mode". Doubles as a timeout in jiffies:
* a full sync is triggered after this time elapses without any disk activity.
*/
int laptop_mode;
EXPORT_SYMBOL(laptop_mode);
However, when running a filesystem synchronization operation, the system will
add an additional timer to schedule more writes after the laptop_mode delay.
We don’t want the state of the system to change at all while performing
hibernation, so we cancel those timers.
mm/page-writeback.c:2198
/*
* We're in laptop mode and we've just synced. The sync's writes will have
* caused another writeback to be scheduled by laptop_io_completion.
* Nothing needs to be written back anymore, so we unschedule the writeback.
*/
void laptop_sync_completion(void)
{
struct backing_dev_info *bdi;
rcu_read_lock();
list_for_each_entry_rcu(bdi, &bdi_list, bdi_list)
del_timer(&bdi->laptop_mode_wb_timer);
rcu_read_unlock();
}
As a side note, the ksys_sync function is simply called when the
system call sync is used.
fs/sync.c:111
SYSCALL_DEFINE0(sync)
{
ksys_sync();
return 0;
}
The End of Preparation
With that the system has finished preparations for hibernation.
This is a somewhat arbitrary cutoff, but next the system will begin
a full freeze of userspace to then dump memory out to an image and finally
to perform hibernation. All this will be covered in future articles!
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