Re: [patch -mm 16/18] oom: give current access to memory reserves if it has been killed

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> It's possible to livelock the page allocator if a thread has mm->mmap_sem
> and fails to make forward progress because the oom killer selects another
> thread sharing the same ->mm to kill that cannot exit until the semaphore
> is dropped.
> 
> The oom killer will not kill multiple tasks at the same time; each oom
> killed task must exit before another task may be killed.  Thus, if one
> thread is holding mm->mmap_sem and cannot allocate memory, all threads
> sharing the same ->mm are blocked from exiting as well.  In the oom kill
> case, that means the thread holding mm->mmap_sem will never free
> additional memory since it cannot get access to memory reserves and the
> thread that depends on it with access to memory reserves cannot exit
> because it cannot acquire the semaphore.  Thus, the page allocators
> livelocks.
> 
> When the oom killer is called and current happens to have a pending
> SIGKILL, this patch automatically gives it access to memory reserves and
> returns.  Upon returning to the page allocator, its allocation will
> hopefully succeed so it can quickly exit and free its memory.  If not, the
> page allocator will fail the allocation if it is not __GFP_NOFAIL.
> 
> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Signed-off-by: David Rientjes <rientjes@google.com>

ack.



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