lkml.org 
[lkml]   [2017]   [Apr]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [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@virtuozzo.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

\
 
 \ /
  Last update: 2017-04-03 14:46    [W:0.061 / U:0.804 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site