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 >>> <[email protected]> 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 <[email protected]> Date: Thu Jul 28 15:49:19 2016 -0700 mm, page_alloc: restructure direct compaction handling in slowpath
