Re: [PATCH vfs.tmpfs 4/5] tmpfs: trivial support for direct IO

[Jan Kara <jack@suse.cz>]

On Tue 08-08-23 21:34:54, Hugh Dickins wrote:
> Depending upon your philosophical viewpoint, either tmpfs always does
> direct IO, or it cannot ever do direct IO; but whichever, if tmpfs is to
> stand in for a more sophisticated filesystem, it can be helpful for tmpfs
> to support O_DIRECT.  So, give tmpfs a shmem_direct_IO() method, of the
> simplest kind: by just returning 0 done, it leaves all the work to the
> buffered fallback (and everything else just happens to work out okay -
> in particular, its dirty pages don't get lost to invalidation).
> 
> xfstests auto generic which were not run on tmpfs before but now pass:
> 036 091 113 125 130 133 135 198 207 208 209 210 211 212 214 226 239 263
> 323 355 391 406 412 422 427 446 451 465 551 586 591 609 615 647 708 729
> with no new failures.
> 
> LTP dio tests which were not run on tmpfs before but now pass:
> dio01 through dio30, except for dio04 and dio10, which fail because
> tmpfs dio read and write allow odd count: tmpfs could be made stricter,
> but would that be an improvement?
> 
> Signed-off-by: Hugh Dickins <hughd@google.com>

Yeah, we are not quite consistent about whether it is better to silently
fallback to buffered IO or return error among filesystems. So I guess
whatever you like. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  mm/shmem.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 7420b510a9f3..4d5599e566df 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2720,6 +2720,16 @@ shmem_write_end(struct file *file, struct address_space *mapping,
>  	return copied;
>  }
>  
> +static ssize_t shmem_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
> +{
> +	/*
> +	 * Just leave all the work to the buffered fallback.
> +	 * Some LTP tests may expect us to enforce alignment restrictions,
> +	 * but the fallback works just fine with any alignment, so allow it.
> +	 */
> +	return 0;
> +}
> +
>  static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
>  {
>  	struct file *file = iocb->ki_filp;
> @@ -4421,6 +4431,7 @@ const struct address_space_operations shmem_aops = {
>  #ifdef CONFIG_TMPFS
>  	.write_begin	= shmem_write_begin,
>  	.write_end	= shmem_write_end,
> +	.direct_IO	= shmem_direct_IO,
>  #endif
>  #ifdef CONFIG_MIGRATION
>  	.migrate_folio	= migrate_folio,
> -- 
> 2.35.3
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR