Date:   Wed, 7 Sep 2022 11:10:02 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, linux-xfs@...r.kernel.org,
        linux-api@...r.kernel.org, linux-fscrypt@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Keith Busch <kbusch@...nel.org>,
        Christoph Hellwig <hch@....de>,
        "Darrick J. Wong" <djwong@...nel.org>,
        "Martin K. Petersen" <martin.petersen@...cle.com>
Subject: Re: [PATCH v5 1/8] statx: add direct I/O alignment information

On Fri, Aug 26, 2022 at 11:58:44PM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@...gle.com>
> 
> Traditionally, the conditions for when DIO (direct I/O) is supported
> were fairly simple.  For both block devices and regular files, DIO had
> to be aligned to the logical block size of the block device.
> 
> However, due to filesystem features that have been added over time (e.g.
> multi-device support, data journalling, inline data, encryption, verity,
> compression, checkpoint disabling, log-structured mode), the conditions
> for when DIO is allowed on a regular file have gotten increasingly
> complex.  Whether a particular regular file supports DIO, and with what
> alignment, can depend on various file attributes and filesystem mount
> options, as well as which block device(s) the file's data is located on.
> 
> Moreover, the general rule of DIO needing to be aligned to the block
> device's logical block size was recently relaxed to allow user buffers
> (but not file offsets) aligned to the DMA alignment instead.  See
> commit bf8d08532bc1 ("iomap: add support for dma aligned direct-io").
> 
> XFS has an ioctl XFS_IOC_DIOINFO that exposes DIO alignment information.
> Uplifting this to the VFS is one possibility.  However, as discussed
> (https://lore.kernel.org/linux-fsdevel/20220120071215.123274-1-ebiggers@kernel.org/T/#u),
> this ioctl is rarely used and not known to be used outside of
> XFS-specific code.  It was also never intended to indicate when a file
> doesn't support DIO at all, nor was it intended for block devices.
> 
> Therefore, let's expose this information via statx().  Add the
> STATX_DIOALIGN flag and two new statx fields associated with it:
> 
> * stx_dio_mem_align: the alignment (in bytes) required for user memory
>   buffers for DIO, or 0 if DIO is not supported on the file.
> 
> * stx_dio_offset_align: the alignment (in bytes) required for file
>   offsets and I/O segment lengths for DIO, or 0 if DIO is not supported
>   on the file.  This will only be nonzero if stx_dio_mem_align is
>   nonzero, and vice versa.
> 
> Note that as with other statx() extensions, if STATX_DIOALIGN isn't set
> in the returned statx struct, then these new fields won't be filled in.
> This will happen if the file is neither a regular file nor a block
> device, or if the file is a regular file and the filesystem doesn't
> support STATX_DIOALIGN.  It might also happen if the caller didn't
> include STATX_DIOALIGN in the request mask, since statx() isn't required
> to return unrequested information.
> 
> This commit only adds the VFS-level plumbing for STATX_DIOALIGN.  For
> regular files, individual filesystems will still need to add code to
> support it.  For block devices, a separate commit will wire it up too.
> 
> Reviewed-by: Christoph Hellwig <hch@....de>
> Reviewed-by: Darrick J. Wong <djwong@...nel.org>
> Reviewed-by: Martin K. Petersen <martin.petersen@...cle.com>
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>
> ---

Looks good to me,
Reviewed-by: Christian Brauner (Microsoft) <brauner@...nel.org>

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