On Wed, Oct 27, 2021 at 04:28:03AM -0400, Mikulas Patocka wrote: > > > 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/ Ok, you need to keep a changelog with the patch so that it's clear what the history of it is.... > ) and Christoph Hellwig asked for a flag in the file_operations structure > ( https://lore.kernel.org/all/20210924155822.GA10064@xxxxxx/ ). Looking at the code that has resulted, I think Christoph's suggestion is a poor one. Code duplication is bad enough, worse is that it's duplicating the open coding of non-trivial flag combinations. Given that it is only needed for a single calling context and it is unnecessary to solve the unique problem at hand (suppress warning and turn off discard support) this makes it seem like a case of over-engineering. Further, it doesn't avoid the need for the loop device to handle EOPNOTSUPP from fallocate directly, either, because as I explained above "filesystem type supports the FALLOC_FL_PUNCH_HOLE API flag" is not the same as "filesystem and/or file instance can execute FALLOC_FL_PUNCH_HOLE".... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
