Re: [PATCH 1/4] mm: prevent potential recursive reclaim due to clearing PF_MEMALLOC

[Vlastimil Babka <vbabka@suse.cz>]

On 04/05/2017 01:40 PM, Andrey Ryabinin wrote:
> 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);
> 
> ?

Well, I didn't care much considering this is for stable only, and patch 2/4
rewrites this to the new api.

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