linux-toradex.git/mm/oom_kill.c, branch v4.0.2

mm: account pmd page tables to the process

2015-02-12T01:06:04+00:00

Dave noticed that unprivileged process can allocate significant amount of
memory -- >500 MiB on x86_64 -- and stay unnoticed by oom-killer and
memory cgroup.  The trick is to allocate a lot of PMD page tables.  Linux
kernel doesn't account PMD tables to the process, only PTE.

The use-cases below use few tricks to allocate a lot of PMD page tables
while keeping VmRSS and VmPTE low.  oom_score for the process will be 0.

	#include 
	#include 
	#include 
	#include 
	#include 
	#include 

	#define PUD_SIZE (1UL << 30)
	#define PMD_SIZE (1UL << 21)

	#define NR_PUD 130000

	int main(void)
	{
		char *addr = NULL;
		unsigned long i;

		prctl(PR_SET_THP_DISABLE);
		for (i = 0; i < NR_PUD ; i++) {
			addr = mmap(addr + PUD_SIZE, PUD_SIZE, PROT_WRITE|PROT_READ,
					MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
			if (addr == MAP_FAILED) {
				perror("mmap");
				break;
			}
			*addr = 'x';
			munmap(addr, PMD_SIZE);
			mmap(addr, PMD_SIZE, PROT_WRITE|PROT_READ,
					MAP_ANONYMOUS|MAP_PRIVATE|MAP_FIXED, -1, 0);
			if (addr == MAP_FAILED)
				perror("re-mmap"), exit(1);
		}
		printf("PID %d consumed %lu KiB in PMD page tables\n",
				getpid(), i * 4096 >> 10);
		return pause();
	}

The patch addresses the issue by account PMD tables to the process the
same way we account PTE.

The main place where PMD tables is accounted is __pmd_alloc() and
free_pmd_range(). But there're few corner cases:

 - HugeTLB can share PMD page tables. The patch handles by accounting
   the table to all processes who share it.

 - x86 PAE pre-allocates few PMD tables on fork.

 - Architectures with FIRST_USER_ADDRESS > 0. We need to adjust sanity
   check on exit(2).

Accounting only happens on configuration where PMD page table's level is
present (PMD is not folded).  As with nr_ptes we use per-mm counter.  The
counter value is used to calculate baseline for badness score by
oom-killer.

Signed-off-by: Kirill A. Shutemov 
Reported-by: Dave Hansen 
Cc: Hugh Dickins 
Reviewed-by: Cyrill Gorcunov 
Cc: Pavel Emelyanov 
Cc: David Rientjes 
Tested-by: Sedat Dilek 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

oom, PM: make OOM detection in the freezer path raceless

2015-02-12T01:06:03+00:00

Commit 5695be142e20 ("OOM, PM: OOM killed task shouldn't escape PM
suspend") has left a race window when OOM killer manages to
note_oom_kill after freeze_processes checks the counter.  The race
window is quite small and really unlikely and partial solution deemed
sufficient at the time of submission.

Tejun wasn't happy about this partial solution though and insisted on a
full solution.  That requires the full OOM and freezer's task freezing
exclusion, though.  This is done by this patch which introduces oom_sem
RW lock and turns oom_killer_disable() into a full OOM barrier.

oom_killer_disabled check is moved from the allocation path to the OOM
level and we take oom_sem for reading for both the check and the whole
OOM invocation.

oom_killer_disable() takes oom_sem for writing so it waits for all
currently running OOM killer invocations.  Then it disable all the further
OOMs by setting oom_killer_disabled and checks for any oom victims.
Victims are counted via mark_tsk_oom_victim resp.  unmark_oom_victim.  The
last victim wakes up all waiters enqueued by oom_killer_disable().
Therefore this function acts as the full OOM barrier.

The page fault path is covered now as well although it was assumed to be
safe before.  As per Tejun, "We used to have freezing points deep in file
system code which may be reacheable from page fault." so it would be
better and more robust to not rely on freezing points here.  Same applies
to the memcg OOM killer.

out_of_memory tells the caller whether the OOM was allowed to trigger and
the callers are supposed to handle the situation.  The page allocation
path simply fails the allocation same as before.  The page fault path will
retry the fault (more on that later) and Sysrq OOM trigger will simply
complain to the log.

Normally there wouldn't be any unfrozen user tasks after
try_to_freeze_tasks so the function will not block. But if there was an
OOM killer racing with try_to_freeze_tasks and the OOM victim didn't
finish yet then we have to wait for it. This should complete in a finite
time, though, because

	- the victim cannot loop in the page fault handler (it would die
	  on the way out from the exception)
	- it cannot loop in the page allocator because all the further
	  allocation would fail and __GFP_NOFAIL allocations are not
	  acceptable at this stage
	- it shouldn't be blocked on any locks held by frozen tasks
	  (try_to_freeze expects lockless context) and kernel threads and
	  work queues are not frozen yet

Signed-off-by: Michal Hocko 
Suggested-by: Tejun Heo 
Cc: David Rientjes 
Cc: Johannes Weiner 
Cc: Oleg Nesterov 
Cc: Cong Wang 
Cc: "Rafael J. Wysocki" 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

oom: thaw the OOM victim if it is frozen

2015-02-12T01:06:03+00:00

oom_kill_process only sets TIF_MEMDIE flag and sends a signal to the
victim.  This is basically noop when the task is frozen though because the
task sleeps in the uninterruptible sleep.  The victim is eventually thawed
later when oom_scan_process_thread meets the task again in a later OOM
invocation so the OOM killer doesn't live lock.  But this is less than
optimal.

Let's add __thaw_task into mark_tsk_oom_victim after we set TIF_MEMDIE to
the victim.  We are not checking whether the task is frozen because that
would be racy and __thaw_task does that already.  oom_scan_process_thread
doesn't need to care about freezer anymore as TIF_MEMDIE and freezer are
excluded completely now.

Signed-off-by: Michal Hocko 
Cc: Tejun Heo 
Cc: David Rientjes 
Cc: Johannes Weiner 
Cc: Oleg Nesterov 
Cc: Cong Wang 
Cc: "Rafael J. Wysocki" 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

oom: add helpers for setting and clearing TIF_MEMDIE

2015-02-12T01:06:03+00:00

This patchset addresses a race which was described in the changelog for
5695be142e20 ("OOM, PM: OOM killed task shouldn't escape PM suspend"):

: PM freezer relies on having all tasks frozen by the time devices are
: getting frozen so that no task will touch them while they are getting
: frozen.  But OOM killer is allowed to kill an already frozen task in order
: to handle OOM situtation.  In order to protect from late wake ups OOM
: killer is disabled after all tasks are frozen.  This, however, still keeps
: a window open when a killed task didn't manage to die by the time
: freeze_processes finishes.

The original patch hasn't closed the race window completely because that
would require a more complex solution as it can be seen by this patchset.

The primary motivation was to close the race condition between OOM killer
and PM freezer _completely_.  As Tejun pointed out, even though the race
condition is unlikely the harder it would be to debug weird bugs deep in
the PM freezer when the debugging options are reduced considerably.  I can
only speculate what might happen when a task is still runnable
unexpectedly.

On a plus side and as a side effect the oom enable/disable has a better
(full barrier) semantic without polluting hot paths.

I have tested the series in KVM with 100M RAM:
- many small tasks (20M anon mmap) which are triggering OOM continually
- s2ram which resumes automatically is triggered in a loop
	echo processors > /sys/power/pm_test
	while true
	do
		echo mem > /sys/power/state
		sleep 1s
	done
- simple module which allocates and frees 20M in 8K chunks. If it sees
  freezing(current) then it tries another round of allocation before calling
  try_to_freeze
- debugging messages of PM stages and OOM killer enable/disable/fail added
  and unmark_oom_victim is delayed by 1s after it clears TIF_MEMDIE and before
  it wakes up waiters.
- rebased on top of the current mmotm which means some necessary updates
  in mm/oom_kill.c. mark_tsk_oom_victim is now called under task_lock but
  I think this should be OK because __thaw_task shouldn't interfere with any
  locking down wake_up_process. Oleg?

As expected there are no OOM killed tasks after oom is disabled and
allocations requested by the kernel thread are failing after all the tasks
are frozen and OOM disabled.  I wasn't able to catch a race where
oom_killer_disable would really have to wait but I kinda expected the race
is really unlikely.

[  242.609330] Killed process 2992 (mem_eater) total-vm:24412kB, anon-rss:2164kB, file-rss:4kB
[  243.628071] Unmarking 2992 OOM victim. oom_victims: 1
[  243.636072] (elapsed 2.837 seconds) done.
[  243.641985] Trying to disable OOM killer
[  243.643032] Waiting for concurent OOM victims
[  243.644342] OOM killer disabled
[  243.645447] Freezing remaining freezable tasks ... (elapsed 0.005 seconds) done.
[  243.652983] Suspending console(s) (use no_console_suspend to debug)
[  243.903299] kmem_eater: page allocation failure: order:1, mode:0x204010
[...]
[  243.992600] PM: suspend of devices complete after 336.667 msecs
[  243.993264] PM: late suspend of devices complete after 0.660 msecs
[  243.994713] PM: noirq suspend of devices complete after 1.446 msecs
[  243.994717] ACPI: Preparing to enter system sleep state S3
[  243.994795] PM: Saving platform NVS memory
[  243.994796] Disabling non-boot CPUs ...

The first 2 patches are simple cleanups for OOM.  They should go in
regardless the rest IMO.

Patches 3 and 4 are trivial printk -> pr_info conversion and they should
go in ditto.

The main patch is the last one and I would appreciate acks from Tejun and
Rafael.  I think the OOM part should be OK (except for __thaw_task vs.
task_lock where a look from Oleg would appreciated) but I am not so sure I
haven't screwed anything in the freezer code.  I have found several
surprises there.

This patch (of 5):

This patch is just a preparatory and it doesn't introduce any functional
change.

Note:
I am utterly unhappy about lowmemory killer abusing TIF_MEMDIE just to
wait for the oom victim and to prevent from new killing. This is
just a side effect of the flag. The primary meaning is to give the oom
victim access to the memory reserves and that shouldn't be necessary
here.

Signed-off-by: Michal Hocko 
Cc: Tejun Heo 
Cc: David Rientjes 
Cc: Johannes Weiner 
Cc: Oleg Nesterov 
Cc: Cong Wang 
Cc: "Rafael J. Wysocki" 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

oom: make sure that TIF_MEMDIE is set under task_lock

2015-02-12T01:06:00+00:00

OOM killer tries to exclude tasks which do not have mm_struct associated
because killing such a task wouldn't help much.  The OOM victim gets
TIF_MEMDIE set to disable OOM killer while the current victim releases the
memory and then enables the OOM killer again by dropping the flag.

oom_kill_process is currently prone to a race condition when the OOM
victim is already exiting and TIF_MEMDIE is set after the task releases
its address space.  This might theoretically lead to OOM livelock if the
OOM victim blocks on an allocation later during exiting because it
wouldn't kill any other process and the exiting one won't be able to exit.
 The situation is highly unlikely because the OOM victim is expected to
release some memory which should help to sort out OOM situation.

Fix this by checking task->mm and setting TIF_MEMDIE flag under task_lock
which will serialize the OOM killer with exit_mm which sets task->mm to
NULL.  Setting the flag for current is not necessary because check and set
is not racy.

Reported-by: Tetsuo Handa 
Signed-off-by: Michal Hocko 
Cc: David Rientjes 
Cc: Oleg Nesterov 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

oom: don't count on mm-less current process

2015-02-12T01:06:00+00:00

out_of_memory() doesn't trigger the OOM killer if the current task is
already exiting or it has fatal signals pending, and gives the task
access to memory reserves instead.  However, doing so is wrong if
out_of_memory() is called by an allocation (e.g. from exit_task_work())
after the current task has already released its memory and cleared
TIF_MEMDIE at exit_mm().  If we again set TIF_MEMDIE to post-exit_mm()
current task, the OOM killer will be blocked by the task sitting in the
final schedule() waiting for its parent to reap it.  It will trigger an
OOM livelock if its parent is unable to reap it due to doing an
allocation and waiting for the OOM killer to kill it.

Signed-off-by: Tetsuo Handa 
Acked-by: Michal Hocko 
Cc: David Rientjes 
Cc: Oleg Nesterov 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

oom: kill the insufficient and no longer needed PT_TRACE_EXIT check

2014-12-13T20:42:49+00:00

After the previous patch we can remove the PT_TRACE_EXIT check in
oom_scan_process_thread(), it was added to handle the case when the
coredumping was "frozen" by ptrace, but it doesn't really work.  If
nothing else, we would need to check all threads which could share the
same ->mm to make it more or less correct.

Signed-off-by: Oleg Nesterov 
Cc: Cong Wang 
Cc: David Rientjes 
Acked-by: Michal Hocko 
Cc: "Rafael J. Wysocki" 
Cc: Tejun Heo 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

oom: don't assume that a coredumping thread will exit soon

2014-12-13T20:42:49+00:00

oom_kill.c assumes that PF_EXITING task should exit and free the memory
soon.  This is wrong in many ways and one important case is the coredump.
A task can sleep in exit_mm() "forever" while the coredumping sub-thread
can need more memory.

Change the PF_EXITING checks to take SIGNAL_GROUP_COREDUMP into account,
we add the new trivial helper for that.

Note: this is only the first step, this patch doesn't try to solve other
problems.  The SIGNAL_GROUP_COREDUMP check is obviously racy, a task can
participate in coredump after it was already observed in PF_EXITING state,
so TIF_MEMDIE (which also blocks oom-killer) still can be wrongly set.
fatal_signal_pending() can be true because of SIGNAL_GROUP_COREDUMP so
out_of_memory() and mem_cgroup_out_of_memory() shouldn't blindly trust it.
 And even the name/usage of the new helper is confusing, an exiting thread
can only free its ->mm if it is the only/last task in thread group.

[akpm@linux-foundation.org: add comment]
Signed-off-by: Oleg Nesterov 
Cc: Cong Wang 
Acked-by: David Rientjes 
Acked-by: Michal Hocko 
Cc: "Rafael J. Wysocki" 
Cc: Tejun Heo 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

Merge branch 'for-3.19' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup

2014-12-12T02:57:19+00:00

Pull cgroup update from Tejun Heo:
 "cpuset got simplified a bit.  cgroup core got a fix on unified
  hierarchy and grew some effective css related interfaces which will be
  used for blkio support for writeback IO traffic which is currently
  being worked on"

* 'for-3.19' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup:
  cgroup: implement cgroup_get_e_css()
  cgroup: add cgroup_subsys->css_e_css_changed()
  cgroup: add cgroup_subsys->css_released()
  cgroup: fix the async css offline wait logic in cgroup_subtree_control_write()
  cgroup: restructure child_subsys_mask handling in cgroup_subtree_control_write()
  cgroup: separate out cgroup_calc_child_subsys_mask() from cgroup_refresh_child_subsys_mask()
  cpuset: lock vs unlock typo
  cpuset: simplify cpuset_node_allowed API
  cpuset: convert callback_mutex to a spinlock

mm: memcontrol: drop bogus RCU locking from mem_cgroup_same_or_subtree()

2014-12-11T01:41:08+00:00

None of the mem_cgroup_same_or_subtree() callers actually require it to
take the RCU lock, either because they hold it themselves or they have css
references.  Remove it.

To make the API change clear, rename the leftover helper to
mem_cgroup_is_descendant() to match cgroup_is_descendant().

Signed-off-by: Johannes Weiner 
Reviewed-by: Vladimir Davydov 
Acked-by: Michal Hocko 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds