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/[email protected]
Signed-off-by: Vlastimil Babka <[email protected]>
Reported-by: Andrey Ryabinin <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Acked-by: Hillf Danton <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Boris Brezillon <[email protected]>
Cc: Chris Leech <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Josef Bacik <[email protected]>
Cc: Lee Duncan <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Richard Weinberger <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e7486afa7fa743689e057be68a702a0cb11db583..1daf509722c7fc0063bb2823a69ec3d6f4612fe6 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3283,6 +3283,7 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
                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_mask, unsigned int order,
        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;