On Sun 23-08-15 16:21:41, Tetsuo Handa wrote:
> >From 4a3cf5be07a66cf3906a380e77ba5e2ac1b2b3d5 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Date: Sat, 1 Aug 2015 22:39:30 +0900
> Subject: [PATCH 1/2] mm: Fix race between setting TIF_MEMDIE and
> __alloc_pages_high_priority().
>
> Currently, TIF_MEMDIE is checked at gfp_to_alloc_flags() which is before
> calling __alloc_pages_high_priority() and at
>
> /* Avoid allocations with no watermarks from looping endlessly */
> if (test_thread_flag(TIF_MEMDIE) && !(gfp_mask & __GFP_NOFAIL))
>
> which is after returning from __alloc_pages_high_priority(). This means
> that if TIF_MEMDIE is set between returning from gfp_to_alloc_flags() and
> checking test_thread_flag(TIF_MEMDIE), the allocation can fail without
> calling __alloc_pages_high_priority(). We need to replace
> "test_thread_flag(TIF_MEMDIE)" with "whether TIF_MEMDIE was already set
> as of calling gfp_to_alloc_flags()" in order to close this race window.
>
> Since gfp_to_alloc_flags() includes ALLOC_NO_WATERMARKS for several cases,
> it will be more correct to replace "test_thread_flag(TIF_MEMDIE)" with
> "whether gfp_to_alloc_flags() included ALLOC_NO_WATERMARKS" because the
> purpose of test_thread_flag(TIF_MEMDIE) is to give up immediately if
> __alloc_pages_high_priority() failed.
Yes TIF_MEMDIE setting is inherently racy. We will fail the allocation
without diving into reserves. Why is that a problem?
The comment above the check is misleading but now you are allowing to
fail all ALLOC_NO_WATERMARKS (without __GFP_NOFAIL) allocations before
entering the direct reclaim and compaction. This seems incorrect. What
about __GFP_MEMALLOC requests?
I think the check for TIF_MEMDIE makes more sense here.
>
> Note that we could simply do
>
> if (alloc_flags & ALLOC_NO_WATERMARKS) {
> ac->zonelist = node_zonelist(numa_node_id(), gfp_mask);
> page = __alloc_pages_high_priority(gfp_mask, order, ac);
> if (page)
> goto got_pg;
> WARN_ON_ONCE(!wait && (gfp_mask & __GFP_NOFAIL));
> goto nopage;
> }
>
> instead of changing to
>
> if ((alloc_flags & ALLOC_NO_WATERMARKS) && !(gfp_mask & __GFP_NOFAIL))
> goto nopage;
>
> if we can duplicate
>
> if (!wait) {
> WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL);
> goto nopage;
> }
>
> .
>
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> ---
> mm/page_alloc.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 4b220cb..37a0390 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3085,7 +3085,7 @@ retry:
> goto nopage;
>
> /* Avoid allocations with no watermarks from looping endlessly */
> - if (test_thread_flag(TIF_MEMDIE) && !(gfp_mask & __GFP_NOFAIL))
> + if ((alloc_flags & ALLOC_NO_WATERMARKS) && !(gfp_mask & __GFP_NOFAIL))
> goto nopage;
>
> /*
> --
> 1.8.3.1
--
Michal Hocko
SUSE Labs
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