Re: [PATCH v10 0/5] add support for direct I/O with fscrypt using blk-crypto

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Jan 20, 2022 at 12:30:23AM -0800, Christoph Hellwig wrote:
> On Wed, Jan 19, 2022 at 11:12:10PM -0800, Eric Biggers wrote:
> > 
> > Given the above, as far as I know the only remaining objection to this
> > patchset would be that DIO constraints aren't sufficiently discoverable
> > by userspace.  Now, to put this in context, this is a longstanding issue
> > with all Linux filesystems, except XFS which has XFS_IOC_DIOINFO.  It's
> > not specific to this feature, and it doesn't actually seem to be too
> > important in practice; many other filesystem features place constraints
> > on DIO, and f2fs even *only* allows fully FS block size aligned DIO.
> > (And for better or worse, many systems using fscrypt already have
> > out-of-tree patches that enable DIO support, and people don't seem to
> > have trouble with the FS block size alignment requirement.)
> 
> It might make sense to use this as an opportunity to implement
> XFS_IOC_DIOINFO for ext4 and f2fs.

Hmm.  A potential problem with DIOINFO is that it doesn't explicitly
list the /file/ position alignment requirement:

struct dioattr {
	__u32		d_mem;		/* data buffer memory alignment */
	__u32		d_miniosz;	/* min xfer size		*/
	__u32		d_maxiosz;	/* max xfer size		*/
};

Since I /think/ fscrypt requires that directio writes be aligned to file
block size, right?

> > I plan to propose a new generic ioctl to address the issue of DIO
> > constraints being insufficiently discoverable.  But until then, I'm

Which is what I suspect Eric meant by this sentence. :)

> > wondering if people are willing to consider this patchset again, or
> > whether it is considered blocked by this issue alone.  (And if this
> > patchset is still unacceptable, would it be acceptable with f2fs support
> > only, given that f2fs *already* only allows FS block size aligned DIO?)
> 
> I think the patchset looks fine, but I'd really love to have a way for
> the alignment restrictions to be discoverable from the start.

I agree.  The mechanics of the patchset look ok to me, but it's very
unfortunate that there's no way for userspace programs to ask the kernel
about the directio geometry for a file.

Ever since we added reflink to XFS I've wanted to add a way to tell
userspace that direct writes to a reflink(able) file will be much more
efficient if they can align the io request to 1 fs block instead of 1
sector.

How about something like this:

struct dioattr2 {
	__u32		d_mem;		/* data buffer memory alignment */
	__u32		d_miniosz;	/* min xfer size		*/
	__u32		d_maxiosz;	/* max xfer size		*/

	/* file range must be aligned to this value */
	__u32		d_min_fpos;

	/* for optimal performance, align file range to this */
	__u32		d_opt_fpos;

	__u32		d_padding[11];
};

--D