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
