[dm-devel] [PATCH v8 3/5] loop: Add support for provision requests

[Dave Chinner]

On Fri, Oct 06, 2023 at 06:28:15PM -0700, Sarthak Kukreti wrote:
> Add support for provision requests to loopback devices.
> Loop devices will configure provision support based on
> whether the underlying block device/file can support
> the provision request and upon receiving a provision bio,
> will map it to the backing device/storage. For loop devices
> over files, a REQ_OP_PROVISION request will translate to
> an fallocate mode 0 call on the backing file.
> 
> Signed-off-by: Sarthak Kukreti <sarthakkukreti at chromium.org>
> Signed-off-by: Mike Snitzer <snitzer at kernel.org>


Hmmmm.

This doesn't actually implement the required semantics of
REQ_PROVISION. Yes, it passes the command to the filesystem
fallocate() implementation, but fallocate() at the filesystem level
does not have the same semantics as REQ_PROVISION.

i.e. at the filesystem level, fallocate() only guarantees the next
write to the provisioned range will succeed without ENOSPC, it does
not guarantee *every* write to the range will succeed without
ENOSPC. If someone clones the loop file while it is in use (i.e.
snapshots it via cp --reflink) then all guarantees that the next
write to a provisioned LBA range will succeed without ENOSPC are
voided.

So while this will work for basic testing that the filesystem is
issuing REQ_PROVISION based IO correctly, it can't actually be used
for hosting production filesystems that need full REQ_PROVISION
guarantees when the loop device backing file is independently
shapshotted via FICLONE....

At minimuim, this set of implementation constraints needs tobe
documented somewhere...

-Dave.
-- 
Dave Chinner
david at fromorbit.com