linux-ext4 - Re: [PATCH v8 3/5] loop: Add support for provision requests

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Date:   Mon, 9 Oct 2023 10:37:52 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Sarthak Kukreti <sarthakkukreti@...omium.org>
Cc:     dm-devel@...hat.com, linux-block@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Brian Foster <bfoster@...hat.com>,
        Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Bart Van Assche <bvanassche@...gle.com>,
        "Darrick J. Wong" <djwong@...nel.org>
Subject: Re: [PATCH v8 3/5] loop: Add support for provision requests

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@...omium.org>
> Signed-off-by: Mike Snitzer <snitzer@...nel.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@...morbit.com