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From: David Rientjes on 6 Apr 2010 18:50
On Mon, 5 Apr 2010, Mel Gorman wrote:

> > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > > --- a/mm/page_alloc.c
> > > > +++ b/mm/page_alloc.c
> > > > @@ -1610,13 +1610,21 @@ try_next_zone:
> > > > }
> > > >
> > > > static inline int
> > > > -should_alloc_retry(gfp_t gfp_mask, unsigned int order,
> > > > +should_alloc_retry(struct task_struct *p, gfp_t gfp_mask, unsigned int order,
> > > > unsigned long pages_reclaimed)
> > > > {
> > > > /* Do not loop if specifically requested */
> > > > if (gfp_mask & __GFP_NORETRY)
> > > > return 0;
> > > >
> > > > + /* Loop if specifically requested */
> > > > + if (gfp_mask & __GFP_NOFAIL)
> > > > + return 1;
> > > > +
> > >
> > > Meh, you could have preserved the comment but no biggie.
> > >
> >
> > I'll remember to preserve it when it's proposed.
> >
> > > > + /* Task is killed, fail the allocation if possible */
> > > > + if (fatal_signal_pending(p))
> > > > + return 0;
> > > > +
> > >
> > > Seems reasonable. This will be checked on every major loop in the
> > > allocator slow patch.
> > >
> > > > /*
> > > > * In this implementation, order <= PAGE_ALLOC_COSTLY_ORDER
> > > > * means __GFP_NOFAIL, but that may not be true in other
> > > > @@ -1635,13 +1643,6 @@ should_alloc_retry(gfp_t gfp_mask, unsigned int order,
> > > > if (gfp_mask & __GFP_REPEAT && pages_reclaimed < (1 << order))
> > > > return 1;
> > > >
> > > > - /*
> > > > - * Don't let big-order allocations loop unless the caller
> > > > - * explicitly requests that.
> > > > - */
> > > > - if (gfp_mask & __GFP_NOFAIL)
> > > > - return 1;
> > > > -
> > > > return 0;
> > > > }
> > > >
> > > > @@ -1798,6 +1799,7 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
> > > > if (likely(!(gfp_mask & __GFP_NOMEMALLOC))) {
> > > > if (!in_interrupt() &&
> > > > ((p->flags & PF_MEMALLOC) ||
> > > > + (fatal_signal_pending(p) && (gfp_mask & __GFP_NOFAIL)) ||
> > >
> > > This is a lot less clear. GFP_NOFAIL is rare so this is basically saying
> > > that all threads with a fatal signal pending can ignore watermarks. This
> > > is dangerous because if 1000 threads get killed, there is a possibility
> > > of deadlocking the system.
> > >
> >
> > I don't quite understand the comment, this is only for __GFP_NOFAIL
> > allocations, which you say are rare, so a large number of threads won't be
> > doing this simultaneously.
> >
> > > Why not obey the watermarks and just not retry the loop later and fail
> > > the allocation?
> > >
> >
> > The above check for (fatal_signal_pending(p) && (gfp_mask & __GFP_NOFAIL))
> > essentially oom kills p without invoking the oom killer before direct
> > reclaim is invoked. We know it has a pending SIGKILL and wants to exit,
> > so we allow it to allocate beyond the min watermark to avoid costly
> > reclaim or needlessly killing another task.
> >
>
> Sorry, I typod.
>
> GFP_NOFAIL is rare but this is basically saying that all threads with a
> fatal signal and using NOFAIL can ignore watermarks.
>
> I don't think there is any caller in an exit path will be using GFP_NOFAIL
> as it's most common user is file-system related but it still feels unnecssary
> to check this case on every call to the slow path.
>

Ok, that's reasonable. We already handle this case indirectly in -mm.

oom-give-current-access-to-memory-reserves-if-it-has-been-killed.patch in
-mm makes the oom killer set TIF_MEMDIE for current and return without
killing any other task; it's unnecesary to check if
!test_thread_flag(TIF_MEMDIE) before that since the oom killer will be a
no-op anyway if there exist TIF_MEMDIE threads.

The problem is that the should_alloc_retry() logic isn't checked when the
oom killer is called, we immediately retry instead even if the oom killer
didn't do anything. So if the oom killed task fails to exit because it's
looping in the page allocator, that's going to happen forever since
reclaim has failed and the oom killer can't kill anything else (or it's
__GFP_NOFAIL and __alloc_pages_may_oom() will infinitely loop without ever
returning).

I guess this could potentially deplete memory reserves if too many threads
have fatal signals and the oom killer is constantly invoked, regardless of
__GFP_NOFAIL or not. That's why we have always opted to kill a memory
hogging task instead via a tasklist scan: we want to set TIF_MEMDIE for as
few tasks as possible with a large upside of memory freeing.

I'm wondering if we should check should_alloc_retry() first, it seems like
we could get rid of a few different branches in the oom killer path by
doing so: the comparisons to PAGE_ALLOC_COSTLY_ORDER, __GFP_NORETRY, etc.

> > > > unlikely(test_thread_flag(TIF_MEMDIE))))
> > > > alloc_flags |= ALLOC_NO_WATERMARKS;
> > > > }
> > > > @@ -1812,6 +1814,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> > > > int migratetype)
> > > > {
> > > > const gfp_t wait = gfp_mask & __GFP_WAIT;
> > > > + const gfp_t nofail = gfp_mask & __GFP_NOFAIL;
> > > > struct page *page = NULL;
> > > > int alloc_flags;
> > > > unsigned long pages_reclaimed = 0;
> > > > @@ -1876,7 +1879,7 @@ rebalance:
> > > > goto nopage;
> > > >
> > > > /* Avoid allocations with no watermarks from looping endlessly */
> > > > - if (test_thread_flag(TIF_MEMDIE) && !(gfp_mask & __GFP_NOFAIL))
> > > > + if (test_thread_flag(TIF_MEMDIE) && !nofail)
> > > > goto nopage;
> > > >
> > > > /* Try direct reclaim and then allocating */
> > > > @@ -1888,6 +1891,10 @@ rebalance:
> > > > if (page)
> > > > goto got_pg;
> > > >
> > > > + /* Task is killed, fail the allocation if possible */
> > > > + if (fatal_signal_pending(p) && !nofail)
> > > > + goto nopage;
> > > > +
> > >
> > > Again, I would expect this to be caught by should_alloc_retry().
> > >
> >
> > It is, but only after the oom killer is called. We don't want to
> > needlessly kill another task here when p has already been killed but may
> > not be PF_EXITING yet.
> >
>
> Fair point. How about just checking before __alloc_pages_may_oom() is
> called then? This check will be then in a slower path.

Yeah, that's what
oom-give-current-access-to-memory-reserves-if-it-has-been-killed.patch
effectively does.

> I recognise this means that it is also only checked when direct reclaim
> is failing but there is at least one good reason for it.
>
> With this change, processes that have been sigkilled may now fail allocations
> that they might not have failed before. It would be difficult to trigger
> but here is one possible problem with this change;
>
> 1. System was borderline with some trashing
> 2. User starts program that gobbles up lots of memory on page faults,
> trashing the system further and annoying the user
> 3. User sends SIGKILL
> 4. Process was faulting and returns NULL because fatal signal was pending
> 5. Fault path returns VM_FAULT_OOM
> 6. Arch-specific path (on x86 anyway) calls out_of_memory again because
> VM_FAULT_OOM was returned.
>
> Ho hum, I haven't thought about this before but it's also possible that
> a process that is fauling that gets oom-killed will trigger a cascading
> OOM kill. If the system was heavily trashing, it might mean a large
> number of processes get killed.
>

Pagefault ooms default to killing current first in -mm and only kill
another task if current is unkillable for the architectures that use
pagefault_out_of_memory(); the rest of the architectures such as powerpc
just kill current. So while this scenario is plausible, I don't think
there would be a large number of processes getting killed:
pagefault_out_of_memory() will kill current and give it access to memory
reserves and the oom killer won't perform any needless oom killing while
that is happening.
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