On 04/03/2017 02:38 PM, Andrey Ryabinin wrote: > > > 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
Yeah, looks like previously the code subtly relied on compaction being called only after the PF_MEMALLOC -> goto nopage check and I didn't notice it. Tell me if I should add a check or you plan to send a patch. Thanks! > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to [email protected]. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"[email protected]";> [email protected] </a> >
