On 27 June 2018 at 02:39, Andreas Gruenbacher <agruenba@xxxxxxxxxx> wrote:
> Here's a patch that implements direct I/O for inline data. Direct I/O
> to inline data is a bit weird because it's not direct in the usual
> sense, but since Christoph's been asking for it ...
>
> The usual alignment restrictions to the logical block size of the
> underlying block device still apply. I don't see a reason for changing
> that; the resulting behavior would only become very weird for no
> benefit.
>
> I've tested this against a hacked-up version of gfs2. However, the
> "real" gfs2 will keep falling back to buffered I/O for writes to inline
> data: gfs2 takes a shared lock during direct I/O, and writing to the
> inode under that shared lock is not allowed. Ext4 may become the first
> actual user of this part of the patch.
One further issue is the alignment check in iomap_dio_actor:
> if ((pos | length | align) & ((1 << blkbits) - 1))
> return -EINVAL;
For inline data, iomap->length is set to the file size. iomap_apply
truncates the requested length down to that, so iomap_dio_actor sees
the truncated length instead of the requested length and fails with
-EINVAL. This causes tests like the following to fail (also see
xfstest generic/120):
xfs_io -fd -c 'truncate 300' -c 'pread -v 0 4096' /mnt/test/foo
A possible fix is to change the alignment check in iomap_dio_actor as follows:
- if ((pos | length | align) & ((1 << blkbits) - 1))
+ if ((pos | align) & ((1 << blkbits) - 1))
+ return -EINVAL;
+ if (length & ((1 << blkbits) - 1) &&
+ pos + length != iomap->offset + iomap->length)
return -EINVAL;
Moving the alignment check from iomap_dio_actor to iomap_dio_rw isn't
that easy because iomap->bdev isn't known there.
Any thoughts?
Thanks,
Andreas
