On 04/03/2017 03:37 PM, Andrey Ryabinin wrote:
>
>
> On 04/03/2017 11:47 AM, Michal Hocko wrote:
>> On Fri 31-03-17 10:00:30, Shakeel Butt wrote:
>>> On Fri, Mar 31, 2017 at 8:30 AM, Andrey Ryabinin
>>> <aryabinin@xxxxxxxxxxxxx> wrote:
>>>> zswap_frontswap_store() is called during memory reclaim from
>>>> __frontswap_store() from swap_writepage() from shrink_page_list().
>>>> This may happen in NOFS context, thus zswap shouldn't use __GFP_FS,
>>>> otherwise we may renter into fs code and deadlock.
>>>> zswap_frontswap_store() also shouldn't use __GFP_IO to avoid recursion
>>>> into itself.
>>>>
>>>
>>> Is it possible to enter fs code (or IO) from zswap_frontswap_store()
>>> other than recursive memory reclaim? However recursive memory reclaim
>>> is protected through PF_MEMALLOC task flag. The change seems fine but
>>> IMHO reasoning needs an update. Adding Michal for expert opinion.
>>
>> Yes this is true.
>
> Actually, no. I think we have a bug in allocator which may lead to recursive direct reclaim.
>
> E.g. for costly order allocations (or order > 0 && ac->migratetype != MIGRATE_MOVABLE)
> with __GFP_NOMEMALLOC (gfp_pfmemalloc_allowed() returns false)
> __alloc_pages_slowpath() may call __alloc_pages_direct_compact() and unconditionally clear PF_MEMALLOC:
>
> __alloc_pages_direct_compact():
> ...
> current->flags |= PF_MEMALLOC;
> *compact_result = try_to_compact_pages(gfp_mask, order, alloc_flags, ac,
> prio);
> current->flags &= ~PF_MEMALLOC;
>
>
>
> And later in __alloc_pages_slowpath():
>
> /* Avoid recursion of direct reclaim */
> if (current->flags & PF_MEMALLOC) <=== false
> goto nopage;
>
> /* Try direct reclaim and then allocating */
> page = __alloc_pages_direct_reclaim(gfp_mask, order, alloc_flags, ac,
> &did_some_progress);
>
Seems it was broken by
a8161d1ed6098506303c65b3701dedba876df42a
Author: Vlastimil Babka <vbabka@xxxxxxx>
Date: Thu Jul 28 15:49:19 2016 -0700
mm, page_alloc: restructure direct compaction handling in slowpath
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