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SubjectRe: [PATCH] mm: avoid livelock on !__GFP_FS allocations
On Mon, 24 Oct 2011, Colin Cross wrote:

> Under the following conditions, __alloc_pages_slowpath can loop
> forever:
> gfp_mask & __GFP_WAIT is true
> gfp_mask & __GFP_FS is false
> reclaim and compaction make no progress
> order <= PAGE_ALLOC_COSTLY_ORDER
>

The oom killer is only called for __GFP_FS because we want to ensure that
we don't inadvertently kill something if we didn't have a chance to at
least make a good effort at direct reclaim. There's a very high liklihood
that direct reclaim would succeed with __GFP_FS, so we loop endlessly
waiting for either kswapd to reclaim in the background even though it
might not be able to because of filesystem locks or another allocation
happens in a context that allows reclaim to succeed or oom killing.

For low-order allocations (those at or below PAGE_ALLOC_COSTLY_ORDER)
where fragmentation isn't a huge issue, __GFP_WAIT && !__GFP_FS &&
!did_some_progress makes sense.
> These conditions happen very often during suspend and resume,
> when pm_restrict_gfp_mask() effectively converts all GFP_KERNEL
> allocations into __GFP_WAIT.
>

This is the problem. All allocations now have no chance of ever having
direct reclaim succeed nor the oom killer called. It seems like you would
want pm_restrict_gfp_mask() to also include __GFP_NORETRY and ensure it
can never be called for __GFP_NOFAIL.

> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index fef8dc3..dcd99b3 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2193,6 +2193,10 @@ rebalance:
> }
>
> goto restart;
> + } else {
> + /* If we aren't going to try the OOM killer, give up */
> + if (!(gfp_mask & __GFP_NOFAIL))
> + goto nopage;
> }
> }
>

Nack on this, it is going to cause many very verbose allocation failures
(if !__GFP_NOWARN) when not using suspend because we're not in a context
where we can do sensible reclaim or compaction and presently kswapd can
either reclaim or another allocation will allow low-order amounts of
memory to be reclaimed or the oom killer to free some memory. It would
introduce a regression into page allocation.


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