Re: [PATCH v4 0/9] make statx() return DIO alignment information

[Eric Biggers]

On Fri, Aug 26, 2022 at 01:19:37PM -0400, Jeff Layton wrote:
> On Fri, 2022-07-22 at 00:12 -0700, Eric Biggers wrote:
> > This patchset makes the statx() system call return direct I/O (DIO)
> > alignment information.  This allows userspace to easily determine
> > whether a file supports DIO, and if so with what alignment restrictions.
> > 
> > Patch 1 adds the basic VFS support for STATX_DIOALIGN.  Patch 2 wires it
> > up for all block device files.  The remaining patches wire it up for
> > regular files on ext4, f2fs, and xfs.  Support for regular files on
> > other filesystems can be added later.
> > 
> > I've also written a man-pages patch, which I'm sending separately.
> > 
> > Note, f2fs has one corner case where DIO reads are allowed but not DIO
> > writes.  The proposed statx fields can't represent this.  My proposal
> > (patch 6) is to just eliminate this case, as it seems much too weird.
> > But I'd appreciate any feedback on that part.
> > 
> > This patchset applies to v5.19-rc7.
> > 
> > Changed v3 => v4:
> >    - Added xfs support.
> > 
> >    - Moved the helper function for block devices into block/bdev.c.
> >    
> >    - Adjusted the ext4 patch to not introduce a bug where misaligned DIO
> >      starts being allowed on encrypted files when it gets combined with
> >      the patch "iomap: add support for dma aligned direct-io" that is
> >      queued in the block tree for 5.20.
> > 
> >    - Made a simplification in fscrypt_dio_supported().
> > 
> > Changed v2 => v3:
> >    - Dropped the stx_offset_align_optimal field, since its purpose
> >      wasn't clearly distinguished from the existing stx_blksize.
> > 
> >    - Renamed STATX_IOALIGN to STATX_DIOALIGN, to reflect the new focus
> >      on DIO only.
> > 
> >    - Similarly, renamed stx_{mem,offset}_align_dio to
> >      stx_dio_{mem,offset}_align, to reflect the new focus on DIO only.
> > 
> >    - Wired up STATX_DIOALIGN on block device files.
> > 
> > Changed v1 => v2:
> >    - No changes.
> > 
> > Eric Biggers (9):
> >   statx: add direct I/O alignment information
> >   vfs: support STATX_DIOALIGN on block devices
> >   fscrypt: change fscrypt_dio_supported() to prepare for STATX_DIOALIGN
> >   ext4: support STATX_DIOALIGN
> >   f2fs: move f2fs_force_buffered_io() into file.c
> >   f2fs: don't allow DIO reads but not DIO writes
> >   f2fs: simplify f2fs_force_buffered_io()
> >   f2fs: support STATX_DIOALIGN
> >   xfs: support STATX_DIOALIGN
> > 
> >  block/bdev.c              | 25 ++++++++++++++++++++
> >  fs/crypto/inline_crypt.c  | 49 +++++++++++++++++++--------------------
> >  fs/ext4/ext4.h            |  1 +
> >  fs/ext4/file.c            | 37 ++++++++++++++++++++---------
> >  fs/ext4/inode.c           | 36 ++++++++++++++++++++++++++++
> >  fs/f2fs/f2fs.h            | 45 -----------------------------------
> >  fs/f2fs/file.c            | 45 ++++++++++++++++++++++++++++++++++-
> >  fs/stat.c                 | 14 +++++++++++
> >  fs/xfs/xfs_iops.c         |  9 +++++++
> >  include/linux/blkdev.h    |  4 ++++
> >  include/linux/fscrypt.h   |  7 ++----
> >  include/linux/stat.h      |  2 ++
> >  include/uapi/linux/stat.h |  4 +++-
> >  13 files changed, 190 insertions(+), 88 deletions(-)
> > 
> > base-commit: ff6992735ade75aae3e35d16b17da1008d753d28
> 
> Hi Eric,
> 
> Can I ask what your plans are with this set? I didn't see it in
> linux-next yet, so I wasn't sure when you were looking to get it merged.
> I'm working on patches to add a new statx field for the i_version
> counter as well and I want to make sure that our work doesn't collide.
> 

I've just sent v5.  I guess I'll try to get it merged for 6.1.  We were a bit
stuck on the read-only DIO issue.  All things considered though, including that
Christoph thinks it's possible for f2fs to support DIO writes on zoned block
devices, I'm willing to bet that read-only DIO doesn't really matter enough for
it to be worth it to add a direction field to STATX_DIOALIGN (which would make
it harder to use STATX_DIOALIGN, as the field would always have to be checked).

- Eric