On Wed 05-04-17 09:46:57, Vlastimil Babka wrote:
> The function __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")
> Reported-by: Andrey Ryabinin <aryabi...@virtuozzo.com>
> Signed-off-by: Vlastimil Babka <vba...@suse.cz>
> Cc: <sta...@vger.kernel.org>

Acked-by: Michal Hocko <mho...@suse.com>

> ---
>  mm/page_alloc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3589f8be53be..b84e6ffbe756 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3288,6 +3288,7 @@ __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;
> @@ -3295,7 +3296,7 @@ __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;
> -- 
> 2.12.2

-- 
Michal Hocko
SUSE Labs

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