Re: [REPOST] [PATCH 1/2] mm: Fix race between setting TIF_MEMDIE and __alloc_pages_high_priority().
From: Michal Hocko <mhocko@kernel.org>
Date: 2015-08-24 10:03:22
On Sun 23-08-15 16:21:41, Tetsuo Handa wrote:
quoted
From 4a3cf5be07a66cf3906a380e77ba5e2ac1b2b3d5 Mon Sep 17 00:00:00 2001From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> 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.
quoted hunk ↗ jump to hunk
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@I-love.SAKURA.ne.jp> --- 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>