Re: [PATCH v4 4/9] ext4: support STATX_DIOALIGN

Theodore Ts'o
2022-07-22T17:06:05Z

On Fri, Jul 22, 2022 at 12:12:23AM -0700, Eric Biggers wrote:
> -static bool ext4_dio_supported(struct kiocb *iocb, struct iov_iter *iter)
> +/*
> + * Returns %true if the given DIO request should be attempted with DIO, or
> + * %false if it should fall back to buffered I/O.
> + *
> + * DIO isn't well specified; when it's unsupported (either due to the request
> + * being misaligned, or due to the file not supporting DIO at all), filesystems
> + * either fall back to buffered I/O or return EINVAL. For files that don't use
> + * any special features like encryption or verity, ext4 has traditionally
> + * returned EINVAL for misaligned DIO. iomap_dio_rw() uses this convention too.
> + * In this case, we should attempt the DIO, *not* fall back to buffered I/O.
> + *
> + * In contrast, in cases where DIO is unsupported due to ext4 features, ext4
> + * traditionally falls back to buffered I/O.
> + *
> + * This function implements the traditional ext4 behavior in all these cases.

Heh. I had been under the impression that misaligned I/O fell back to
buffered I/O for ext4, since that's what a lot of historical Unix
systems did. Obviously, it's not something I've tested since "you
should never do that".

There's actually some interesting discussion about what Linux *should*
be doing in the futre in this discussion:

https://patchwork.ozlabs.org/project/linux-ext4/patch/1461472078-20104-1-git-send-email-tytso@mit.edu/

Including the following from Christoph Hellwig:

https://patchwork.ozlabs.org/project/linux-ext4/patch/1461472078-20104-1-git-send-email-tytso@mit.edu/#1335016

> I've been doing an audit of our direct I/O implementations, and most
> of them does some form of transparent fallback, including some that
> only pretend to support O_DIRECT, but do anything special for it at all,
> while at the same time we go through greast efforts to check a file
> system actualy supports direct I/O, leading to nasty no-op ->direct_IO
> implementations as we even got that abstraction wrong.
>
> At this point I wonder if we should simply treat O_DIRECT as a hint
> and always allow it, and just let the file system optimize for it
> (skip buffering, require alignment, relaxed Posix atomicy requirements)
> if it is set.

The thread also mentioned XFS_IOC_DIOINFO and how We Really Should
have something with equivalent functionality to the VFS --- six years
ago. :-)

Anyway, this change to ext4 looks good.

Acked-by: Theodore Ts'o <tytso@mit.edu>

- Ted