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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: Sat, 27 Jun 2020 09:09:09 -0400 (EDT)	[thread overview]
Message-ID: <alpine.LRH.2.02.2006270848540.14350@file01.intranet.prod.int.rdu2.redhat.com> (raw)
In-Reply-To: <20200626230847.GI2005@dread.disaster.area>



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.

> Given that the loop device IO path already operates under
> memalloc_noio context, (i.e. the recursion restriction is applied in
> only the context that needs is) I see no reason for making that a
> global reclaim limitation....

I think this is a problem.

Suppose that a filesystem does GFP_NOFS allocation, the allocation 
triggers an IO and waits for it to finish, the loop device driver 
redirects the IO to the same filesystem that did the GFP_NOFS allocation.

I saw this deadlock in the past in the dm-bufio subsystem - see the commit 
9d28eb12447ee08bb5d1e8bb3195cf20e1ecd1c0 that fixed it.

Other subsystems that do IO in GFP_NOFS context may deadlock just like 
bufio.

Mikulas



  reply	other threads:[~2020-06-27 13:16 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 [this message]
2020-06-29  0:35       ` Dave Chinner
2020-06-29 13:43         ` Mikulas Patocka
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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