On Wed, 27 Oct 2021, Dave Chinner wrote: > On Wed, Oct 13, 2021 at 05:28:36AM -0400, Mikulas Patocka wrote: > > Hi > > > > Here I'm sending version 4 of the patch. It adds #include <linux/falloc.h> > > to cifs and overlayfs to fix the bugs found out by the kernel test robot. > > > > Mikulas > > > > > > > > From: Mikulas Patocka <mpatocka@xxxxxxxxxx> > > > > The loop driver checks for the fallocate method and if it is present, it > > assumes that the filesystem can do FALLOC_FL_ZERO_RANGE and > > FALLOC_FL_PUNCH_HOLE requests. However, some filesystems (such as fat, or > > tmpfs) have the fallocate method, but lack the capability to do > > FALLOC_FL_ZERO_RANGE and/or FALLOC_FL_PUNCH_HOLE. > > This seems like a loopback driver level problem, not something > filesystems need to solve. fallocate() is defined to return > -EOPNOTSUPP if a flag is passed that it does not support and that's > the mechanism used to inform callers that a fallocate function is > not supported by the underlying filesystem/storage. > > Indeed, filesystems can support hole punching at the ->fallocate(), > but then return EOPNOTSUPP because certain dynamic conditions are > not met e.g. CIFS needs sparse file support on the server to support > hole punching, but we don't know this until we actually try to > sparsify the file. IOWs, this patch doesn't address all the cases > where EOPNOTSUPP might actually get returned from filesystems and/or > storage. > > > This results in syslog warnings "blk_update_request: operation not > > supported error, dev loop0, sector 0 op 0x9:(WRITE_ZEROES) flags 0x800800 > > phys_seg 0 prio class 0". The error can be reproduced with this command: > > "truncate -s 1GiB /tmp/file; losetup /dev/loop0 /tmp/file; blkdiscard -z > > /dev/loop0" > > Which I'm assuming comes from this: > > if (unlikely(error && !blk_rq_is_passthrough(req) && > !(req->rq_flags & RQF_QUIET))) > print_req_error(req, error, __func__); > > Which means we could supress the error message quite easily in > lo_fallocate() by doing: > > out: > if (ret == -EOPNOTSUPP) > rq->rq_flags |= RQF_QUIET; > return ret; I did this (see https://lore.kernel.org/all/alpine.LRH.2.02.2109231539520.27863@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ ) and Christoph Hellwig asked for a flag in the file_operations structure ( https://lore.kernel.org/all/20210924155822.GA10064@xxxxxx/ ). Mikulas > And then we can also run blk_queue_flag_clear(QUEUE_FLAG_DISCARD) > (and whatever else is needed to kill discards) to turn off future > discard attempts on that loopback device. This way the problem is > just quietly and correctly handled by the loop device and everything > is good... > > Thoughts? > > Cheers, > > Dave. > -- > Dave Chinner > david@xxxxxxxxxxxxx >
