linux-kernel - Re: [PATCH 03/21] fs/bdev: Add atomic write support info to statx

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Date:   Wed, 4 Oct 2023 15:19:54 +0100
From:   John Garry <john.g.garry@...cle.com>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     Dave Chinner <david@...morbit.com>,
        Bart Van Assche <bvanassche@....org>,
        Eric Biggers <ebiggers@...nel.org>, axboe@...nel.dk,
        kbusch@...nel.org, hch@....de, sagi@...mberg.me,
        jejb@...ux.ibm.com, martin.petersen@...cle.com,
        viro@...iv.linux.org.uk, brauner@...nel.org,
        chandan.babu@...cle.com, dchinner@...hat.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, tytso@....edu, jbongio@...gle.com,
        linux-api@...r.kernel.org,
        Prasad Singamsetty <prasad.singamsetty@...cle.com>
Subject: Re: [PATCH 03/21] fs/bdev: Add atomic write support info to statx

On 03/10/2023 16:46, Darrick J. Wong wrote:
>>      stat->result_mask |= STATX_WRITE_ATOMIC;
> The result_mask (which becomes the statx stx_mask) needs to have
> STATX_WRITE_ATOMIC set any time a filesystem responds to
> STATX_WRITE_ATOMIC being set in the request_mask, even if the response
> is "not supported".
> 
> The attributes_mask also needs to have STATX_ATTR_WRITE_ATOMIC set if
> the filesystem+file can support the flag, even if it's not currently set
> for that file.  This should get turned into a generic vfs helper for the
> next fs that wants to support atomic write units:
> 
> static void generic_fill_statx_atomic_writes(struct kstat *stat,
> 		struct block_device *bdev)
> {
> 	u64 min_bytes;
> 
> 	/* Confirm that the fs driver knows about this statx request */
> 	stat->result_mask |= STATX_WRITE_ATOMIC;
> 
> 	/* Confirm that the file attribute is known to the fs. */
> 	stat->attributes_mask |= STATX_ATTR_WRITE_ATOMIC;
> 
> 	/* Fill out the rest of the atomic write fields if supported */
> 	min_bytes = queue_atomic_write_unit_min_bytes(bdev->bd_queue);
> 	if (min_bytes == 0)
> 		return;
> 
> 	stat->atomic_write_unit_min = min_bytes;
> 	stat->atomic_write_unit_max =
> 			queue_atomic_write_unit_max_bytes(bdev->bd_queue);
> 
> 	/* Atomic writes actually supported on this file. */
> 	stat->attributes |= STATX_ATTR_WRITE_ATOMIC;
> }
> 
> and then:
> 
> 	if (request_mask & STATX_WRITE_ATOMIC)
> 		generic_fill_statx_atomic_writes(stat, bdev);
> 
> 

That looks sensible, but, if used by an FS, we would still need a method 
to include extra FS restrictions, like extent alignment as in 15/21.

Thanks,
John