Re: [PATCH] mm/zswap: fix potential deadlock in zswap_frontswap_store()

[Andrey Ryabinin]

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);