[patch 103/114] mm: prevent potential recursive reclaim due to clearing PF_MEMALLOC

[patch 103/114] mm: prevent potential recursive reclaim due to clearing PF_MEMALLOC

[akpm]

From: Vlastimil Babka <vbabka@suse.cz>
Subject: mm: prevent potential recursive reclaim due to clearing PF_MEMALLOC

Patch series "more robust PF_MEMALLOC handling"

This series aims to unify the setting and clearing of PF_MEMALLOC, which
prevents recursive reclaim.  There are some places that clear the flag
unconditionally from current->flags, which may result in clearing a
pre-existing flag.  This already resulted in a bug report that Patch 1
fixes (without the new helpers, to make backporting easier).  Patch 2
introduces the new helpers, modelled after existing memalloc_noio_* and
memalloc_nofs_* helpers, and converts mm core to use them.  Patches 3 and
4 convert non-mm code.



This patch (of 4):

__alloc_pages_direct_compact() sets PF_MEMALLOC to prevent deadlock during
page migration by lock_page() (see the comment in __unmap_and_move()). 
Then it unconditionally clears the flag, which can clear a pre-existing
PF_MEMALLOC flag and result in recursive reclaim.  This was not a problem
until commit a8161d1ed609 ("mm, page_alloc: restructure direct compaction
handling in slowpath"), because direct compation was called only after
direct reclaim, which was skipped when PF_MEMALLOC flag was set.

Even now it's only a theoretical issue, as the new callsite of
__alloc_pages_direct_compact() is reached only for costly orders and when
gfp_pfmemalloc_allowed() is true, which means either __GFP_NOMEMALLOC is
in gfp_flags or in_interrupt() is true.  There is no such known context,
but let's play it safe and make __alloc_pages_direct_compact() robust for
cases where PF_MEMALLOC is already set.

Fixes: a8161d1ed609 ("mm, page_alloc: restructure direct compaction handling in slowpath")
Link: http://lkml.kernel.org/r/20170405074700.29871-2-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reported-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Chris Leech <cleech@redhat.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Josef Bacik <jbacik@fb.com>
Cc: Lee Duncan <lduncan@suse.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/page_alloc.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff -puN mm/page_alloc.c~mm-prevent-potential-recursive-reclaim-due-to-clearing-pf_memalloc mm/page_alloc.c
--- a/mm/page_alloc.c~mm-prevent-potential-recursive-reclaim-due-to-clearing-pf_memalloc
+++ a/mm/page_alloc.c
@@ -3283,6 +3283,7 @@ __alloc_pages_direct_compact(gfp_t gfp_m
 		enum compact_priority prio, enum compact_result *compact_result)
 {
 	struct page *page;
+	unsigned int noreclaim_flag = current->flags & PF_MEMALLOC;
 
 	if (!order)
 		return NULL;
@@ -3290,7 +3291,7 @@ __alloc_pages_direct_compact(gfp_t gfp_m
 	current->flags |= PF_MEMALLOC;
 	*compact_result = try_to_compact_pages(gfp_mask, order, alloc_flags, ac,
 									prio);
-	current->flags &= ~PF_MEMALLOC;
+	current->flags = (current->flags & ~PF_MEMALLOC) | noreclaim_flag;
 
 	if (*compact_result <= COMPACT_INACTIVE)
 		return NULL;
_