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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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