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
