On Tue, Nov 27, 2018 at 03:34:04PM -0800, Evan Green wrote: > Hi Ming, > > On Mon, Nov 26, 2018 at 6:55 PM Ming Lei <tom.leiming@xxxxxxxxx> wrote: > > > > On Tue, Nov 27, 2018 at 2:55 AM Evan Green <evgreen@xxxxxxxxxxxx> wrote: > > > > > > On Tue, Oct 30, 2018 at 4:06 PM Evan Green <evgreen@xxxxxxxxxxxx> wrote: > > > > > > > > If the backing device for a loop device is a block device, > > > > This shouldn't be a very common use case wrt. loop. > > Yeah, I'm starting to gather that. Or maybe I'm just the first one to > mention it on the kernel lists ;) We've used this in our Chrome OS > installer, I believe for many years. Gwendal piped in with a few > reasons we do it this way on the cover letter, but in general I think > it allows us to have a unified set of functions to install to a file, > disk, or prepare an image that may have a different block size than > those on the running system. OK, got it, it makes sense. > > > > > > > then mirror the discard properties of the underlying block > > > > device into the loop device. While in there, differentiate > > > > between REQ_OP_DISCARD and REQ_OP_WRITE_ZEROES, which are > > > > different for block devices, but which the loop device had > > > > just been lumping together. > > > > > > > > Signed-off-by: Evan Green <evgreen@xxxxxxxxxxxx> > > > > > > Any thoughts on this patch? This fixes issues for us when using a loop > > > device backed by a block device, where we see many logs like: > > > > > > [ 372.767286] print_req_error: I/O error, dev loop5, sector 88125696 > > > > Seems not see any explanation about this IO error and the fix in your patch. > > Could you describe it a bit more? > > Sure, I probably should have included more context with the series. > > The loop device always reports that it supports discard, by setting up > the max_discard_sectors and max_write_zeroes_sectors in the blk queue. > When the loop device gets a discard or write-zeroes request, it turns > around and calls fallocate on the underlying device with the > PUNCH_HOLE flag. This makes sense when you're backed by a file and > hoping to just deallocate the space, but may fail when you're backed > by a block device that doesn't support discard, or doesn't write > zeroes to discarded sectors. Weirdly, lo_discard already had some code > for preserving EOPNOTSUPP, but then later the error is smashed into > EIO. Patch 1 pipes out EOPNOTSUPP properly, so it doesn't get squashed > into EIO. > > Patch 2 reflects the discard characteristics of the underlying device > into the loop device. That way, if you're backed by a file or a block > device that does support discard, everything works great, and user > mode can even see and use the correct discard and write zero > granularities. If you're backed by a block device that does not > support discard, this is exposed to user mode, which then usually > avoids calling fallocate, and doesn't feel betrayed that their > requests are unexpectedly failing. Thanks for your detailed explanation, and I think we need to fix it. Thanks, Ming
