From: Mikulas Patocka <email@example.com> To: Dave Chinner <firstname.lastname@example.org> Cc: "Matthew Wilcox (Oracle)" <email@example.com>, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, Jens Axboe <firstname.lastname@example.org>, NeilBrown <email@example.com> Subject: Re: [PATCH 0/6] Overhaul memalloc_no* Date: Mon, 29 Jun 2020 09:43:23 -0400 (EDT) [thread overview] Message-ID: <alpine.LRH.firstname.lastname@example.org> (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
next prev parent 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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