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From: Mikulas Patocka <mpatocka@redhat.com>
To: Dave Chinner <david@fromorbit.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux-xfs@vger.kernel.org, dm-devel@redhat.com,
	Jens Axboe <axboe@kernel.dk>, NeilBrown <neilb@suse.de>
Subject: Re: [PATCH 0/6] Overhaul memalloc_no*
Date: Mon, 29 Jun 2020 09:43:23 -0400 (EDT)	[thread overview]
Message-ID: <alpine.LRH.2.02.2006290918030.11293@file01.intranet.prod.int.rdu2.redhat.com> (raw)
In-Reply-To: <20200629003550.GJ2005@dread.disaster.area>



On Mon, 29 Jun 2020, Dave Chinner wrote:

> On Sat, Jun 27, 2020 at 09:09:09AM -0400, Mikulas Patocka wrote:
> > 
> > 
> > On Sat, 27 Jun 2020, Dave Chinner wrote:
> > 
> > > On Fri, Jun 26, 2020 at 11:02:19AM -0400, Mikulas Patocka wrote:
> > > > Hi
> > > > 
> > > > I suggest to join memalloc_noio and memalloc_nofs into just one flag that 
> > > > prevents both filesystem recursion and i/o recursion.
> > > > 
> > > > Note that any I/O can recurse into a filesystem via the loop device, thus 
> > > > it doesn't make much sense to have a context where PF_MEMALLOC_NOFS is set 
> > > > and PF_MEMALLOC_NOIO is not set.
> > > 
> > > Correct me if I'm wrong, but I think that will prevent swapping from
> > > GFP_NOFS memory reclaim contexts.
> > 
> > Yes.
> > 
> > > IOWs, this will substantially
> > > change the behaviour of the memory reclaim system under sustained
> > > GFP_NOFS memory pressure. Sustained GFP_NOFS memory pressure is
> > > quite common, so I really don't think we want to telling memory
> > > reclaim "you can't do IO at all" when all we are trying to do is
> > > prevent recursion back into the same filesystem.
> > 
> > So, we can define __GFP_ONLY_SWAP_IO and __GFP_IO.
> 
> Uh, why?
> 
> Exactly what problem are you trying to solve here?

This:

1. The filesystem does a GFP_NOFS allocation.
2. The allocation calls directly a dm-bufio shrinker.
3. The dm-bufio shrinker sees that there is __GFP_IO set, so it assumes 
   that it can do I/O. It selects some dirty buffers, writes them back and 
   waits for the I/O to finish.
4. The dirty buffers belong to a loop device.
5. The loop device thread calls the filesystem that did the GFP_NOFS 
   allocation in step 1 (and that is still waiting for the allocation to 
   succeed).

Note that setting PF_MEMALLOC_NOIO on the loop thread won't help with this 
deadlock.

Do you argue that this is a bug in dm-bufio? Or a bug in the kernel? Or 
that it can't happen?

> > I saw this deadlock in the past in the dm-bufio subsystem - see the commit 
> > 9d28eb12447ee08bb5d1e8bb3195cf20e1ecd1c0 that fixed it.
> 
> 2014?
> 
> /me looks closer.
> 
> Hmmm. Only sent to dm-devel, no comments, no review, just merged.
> No surprise that nobody else actually knows about this commit. Well,
> time to review it ~6 years after it was merged....
> 
> | dm-bufio tested for __GFP_IO. However, dm-bufio can run on a loop block
> | device that makes calls into the filesystem. If __GFP_IO is present and
> | __GFP_FS isn't, dm-bufio could still block on filesystem operations if it
> | runs on a loop block device.
> 
> OK, so from an architectural POV, this commit is fundamentally
> broken - block/device layer allocation should not allow relcaim
> recursion into filesystems because filesystems are dependent on
> the block layer making forwards progress. This commit is trying to
> work around the loop device doing GFP_KERNEL/GFP_NOFS context
> allocation back end IO path of the loop device. This part of the
> loop device is a block device, so needs to run under GFP_NOIO
> context.

I agree that it is broken, but it fixes the above deadlock.

Mikulas



  reply	other threads:[~2020-06-29 13:43 UTC|newest]

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-25 11:31 Matthew Wilcox (Oracle)
2020-06-25 11:31 ` [PATCH 1/6] mm: Replace PF_MEMALLOC_NOIO with memalloc_noio Matthew Wilcox (Oracle)
2020-06-25 12:22   ` Michal Hocko
2020-06-25 12:34     ` Matthew Wilcox
2020-06-25 12:42       ` Michal Hocko
2020-06-25 11:31 ` [PATCH 2/6] mm: Add become_kswapd and restore_kswapd Matthew Wilcox (Oracle)
2020-06-25 12:31   ` Michal Hocko
2020-06-25 11:31 ` [PATCH 3/6] xfs: Convert to memalloc_nofs_save Matthew Wilcox (Oracle)
2020-06-25 11:31 ` [PATCH 4/6] mm: Replace PF_MEMALLOC_NOFS with memalloc_nofs Matthew Wilcox (Oracle)
2020-06-25 13:35   ` Michal Hocko
2020-06-25 11:31 ` [PATCH 5/6] mm: Replace PF_MEMALLOC_NOIO with memalloc_nocma Matthew Wilcox (Oracle)
2020-06-25 11:31 ` [PATCH 6/6] mm: Add memalloc_nowait Matthew Wilcox (Oracle)
2020-06-25 12:40   ` Michal Hocko
2020-06-25 13:10     ` Matthew Wilcox
2020-06-25 13:34       ` Michal Hocko
2020-06-25 19:05   ` kernel test robot
2020-06-25 23:51   ` kernel test robot
2020-06-29  5:08   ` Mike Rapoport
2020-06-29 12:18     ` Matthew Wilcox
2020-06-29 12:52       ` Michal Hocko
2020-06-29 13:45         ` Mike Rapoport
2020-06-29 21:28           ` Matthew Wilcox
2020-06-30  6:34             ` Michal Hocko
2020-07-01  4:12               ` Matthew Wilcox
2020-07-01  5:53                 ` Michal Hocko
2020-07-01  7:04                   ` Mike Rapoport
2020-09-24  0:39   ` Mike Snitzer
2020-09-24  1:10     ` Matthew Wilcox
2020-10-23 14:49   ` Daniel Vetter
2020-06-25 18:48 ` [PATCH 0/6] Overhaul memalloc_no* Darrick J. Wong
2020-06-25 20:34   ` Matthew Wilcox
2020-06-25 20:36   ` Michal Hocko
2020-06-25 20:40     ` Matthew Wilcox
2020-06-26 15:02 ` Mikulas Patocka
2020-06-26 23:08   ` Dave Chinner
2020-06-27 13:09     ` Mikulas Patocka
2020-06-29  0:35       ` Dave Chinner
2020-06-29 13:43         ` Mikulas Patocka [this message]
2020-06-29 22:34           ` Dave Chinner
2020-07-03 14:26             ` [PATCH] dm-bufio: do cleanup from a workqueue Mikulas Patocka
2020-06-29  8:22     ` [PATCH 0/6] Overhaul memalloc_no* Michal Hocko

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