linux-kernel - Re: [PATCH] mm/zswap: fix potential deadlock in zswap_frontswap

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Date:   Mon, 3 Apr 2017 14:45:44 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Shakeel Butt <shakeelb@...gle.com>,
        Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm/zswap: fix potential deadlock in
 zswap_frontswap_store()

On Mon 03-04-17 15:37:07, 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@...tuozzo.com> 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:

Not sure what is the bug here. __GFP_NOMEMALLOC is supposed to inhibit
PF_MEMALLOC. And we do not recurse to the reclaim path. We only do the
compaction. Or what am I missing?

-- 
Michal Hocko
SUSE Labs