Subject: Re: [PATCH 1/4] mm: prevent potential recursive reclaim due to
 clearing PF_MEMALLOC
To: Vlastimil Babka <vbabka@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>
References: <20170405074700.29871-1-vbabka@suse.cz>
 <20170405074700.29871-2-vbabka@suse.cz>
CC: <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        Michal Hocko <mhocko@kernel.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Johannes Weiner <hannes@cmpxchg.org>, <linux-block@vger.kernel.org>,
        <nbd-general@lists.sourceforge.net>, <open-iscsi@googlegroups.com>,
        <linux-scsi@vger.kernel.org>, <netdev@vger.kernel.org>,
        <stable@vger.kernel.org>
From: Andrey Ryabinin <aryabinin@virtuozzo.com>
Message-ID: <1f26f654-abaf-3878-6abb-5e27ff3a289e@virtuozzo.com>
Date: Wed, 5 Apr 2017 14:40:43 +0300
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On 04/05/2017 10:46 AM, 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
                           is false			

> 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@virtuozzo.com>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> Cc: <stable@vger.kernel.org>
> ---
>  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;

Perhaps this would look better:

	tsk_restore_flags(current, noreclaim_flag, PF_MEMALLOC);

?

>  	if (*compact_result <= COMPACT_INACTIVE)
>  		return NULL;
>