On Thu, 5 Sep 2024, Eric Biggers wrote: > On Thu, Sep 05, 2024 at 08:21:46PM +0200, Mikulas Patocka wrote: > > > > > > On Tue, 3 Sep 2024, Eric Biggers wrote: > > > > > From: Eric Biggers <ebiggers@xxxxxxxxxx> > > > > > > Since dm-verity doesn't support writes, the kernel's memory reclaim code > > > will never wait on dm-verity work. That makes the use of WQ_MEM_RECLAIM > > > in dm-verity unnecessary. WQ_MEM_RECLAIM has been present from the > > > beginning of dm-verity, but I could not find a justification for it; > > > I suspect it was just copied from dm-crypt which does support writes. > > > > > > Therefore, remove WQ_MEM_RECLAIM from dm-verity. This eliminates the > > > creation of an unnecessary rescuer thread per dm-verity device. > > > > > > Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx> > > > > Hmm. I can think about a case where you have read-only dm-verity device, > > on the top of that you have dm-snapshot device and on the top of that you > > have a writable filesystem. > > > > When the filesystem needs to write data, it submits some write bios. When > > dm-snapshot receives these write bios, it will read from the dm-verity > > device and write to the snapshot's exception store device. So, dm-verity > > needs WQ_MEM_RECLAIM in this case. > > > > Mikulas > > > > Yes, unfortunately that sounds correct. > > This means that any workqueue involved in fulfilling block device I/O, > regardless of whether that I/O is read or write, has to use WQ_MEM_RECLAIM. > > I wonder if there's any way to safely share the rescuer threads. > > - Eric When I thought about it, I think that removing WQ_MEM_RECLAIM would be incorrect even without snapshot and it could deadlock even with a read-only filesystem directly on the top of dm-verity. There is a limited number of workqueue kernel threads in the system. If all the workqueue kernel threads are busy trying to read some data from a filesystem that is on the top of dm-verity, then the system deadlocks. Dm-verity would wait until one of the work items exits - and the work items would wait for dm-verity to return the data. The probability that this happens is low, but theoretically it is wrong. Mikulas
