Re: [Nbd] [PATCH 1/4] mm: prevent potential recursive reclaim due to clearing PF_MEMALLOC
- To: "'Vlastimil Babka'" <vbabka@...1290...>, "'Andrew Morton'" <akpm@...133...>
- Cc: nbd-general@lists.sourceforge.net, linux-scsi@...25..., netdev@...25..., linux-kernel@...25..., stable@...25..., 'Michal Hocko' <mhocko@...1285...>, linux-block@...25..., linux-mm@...1312..., 'Johannes Weiner' <hannes@...1554...>, 'Andrey Ryabinin' <aryabinin@...2319...>, open-iscsi@...2887..., 'Mel Gorman' <mgorman@...2888...>
- Subject: Re: [Nbd] [PATCH 1/4] mm: prevent potential recursive reclaim due to clearing PF_MEMALLOC
- From: "Hillf Danton" <hillf.zj@...2873...>
- Date: Fri, 07 Apr 2017 15:33:03 +0800
- Message-id: <[🔎] 092301d2af71$31c97fe0$955c7fa0$@alibaba-inc.com>
- Reply-to: Hillf Danton <hillf.zj@...2873...>
- In-reply-to: <20170405074700.29871-2-vbabka@...1290...>
- References: <20170405074700.29871-1-vbabka@...1290...> <20170405074700.29871-2-vbabka@...1290...>
On April 05, 2017 3:47 PM 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 <aryabinin@...2319...>
> Signed-off-by: Vlastimil Babka <vbabka@...1290...>
> Cc: <stable@...25...>
> ---
Acked-by: Hillf Danton <hillf.zj@...2873...>
> 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
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