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

Darrick J. Wong
2022-01-20T17:10:32Z

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