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

Vlastimil Babka
2017-04-07T09:21:45Z

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;
>>
>