Re: [PATCHv6 11/11] iomap: add support for dma aligned direct-io

Keith Busch
2022-07-22T14:44:12Z

On Fri, Jul 22, 2022 at 12:36:01AM -0700, Eric Biggers wrote:
> [+f2fs list and maintainers]
>
> On Fri, Jun 10, 2022 at 12:58:30PM -0700, Keith Busch wrote:
> > From: Keith Busch <kbusch@kernel.org>
> >
> > Use the address alignment requirements from the block_device for direct
> > io instead of requiring addresses be aligned to the block size.
> >
> > Signed-off-by: Keith Busch <kbusch@kernel.org>
> > Reviewed-by: Christoph Hellwig <hch@lst.de>
> > ---
> > fs/iomap/direct-io.c | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> > index 370c3241618a..5d098adba443 100644
> > --- a/fs/iomap/direct-io.c
> > +++ b/fs/iomap/direct-io.c
> > @@ -242,7 +242,6 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
> > struct inode *inode = iter->inode;
> > unsigned int blkbits = blksize_bits(bdev_logical_block_size(iomap->bdev));
> > unsigned int fs_block_size = i_blocksize(inode), pad;
> > - unsigned int align = iov_iter_alignment(dio->submit.iter);
> > loff_t length = iomap_length(iter);
> > loff_t pos = iter->pos;
> > unsigned int bio_opf;
> > @@ -253,7 +252,8 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
> > size_t copied = 0;
> > size_t orig_count;
> >
> > - if ((pos | length | align) & ((1 << blkbits) - 1))
> > + if ((pos | length) & ((1 << blkbits) - 1) ||
> > + !bdev_iter_is_aligned(iomap->bdev, dio->submit.iter))
> > return -EINVAL;
> >
> > if (iomap->type == IOMAP_UNWRITTEN) {
>
> I noticed that this patch is going to break the following logic in
> f2fs_should_use_dio() in fs/f2fs/file.c:
>
> /*
> * Direct I/O not aligned to the disk's logical_block_size will be
> * attempted, but will fail with -EINVAL.
> *
> * f2fs additionally requires that direct I/O be aligned to the
> * filesystem block size, which is often a stricter requirement.
> * However, f2fs traditionally falls back to buffered I/O on requests
> * that are logical_block_size-aligned but not fs-block aligned.
> *
> * The below logic implements this behavior.
> */
> align = iocb->ki_pos | iov_iter_alignment(iter);
> if (!IS_ALIGNED(align, i_blocksize(inode)) &&
> IS_ALIGNED(align, bdev_logical_block_size(inode->i_sb->s_bdev)))
> return false;
>
> return true;
>
> So, f2fs assumes that __iomap_dio_rw() returns an error if the I/O isn't logical
> block aligned. This patch changes that. The result is that DIO will sometimes
> proceed in cases where the I/O doesn't have the fs block alignment required by
> f2fs for all DIO.
>
> Does anyone have any thoughts about what f2fs should be doing here? I think
> it's weird that f2fs has different behaviors for different degrees of
> misalignment: fail with EINVAL if not logical block aligned, else fallback to
> buffered I/O if not fs block aligned. I think it should be one convention or
> the other. Any opinions about which one it should be?

It looks like f2fs just falls back to buffered IO for this condition without
reaching the new code in iomap_dio_bio_iter(). btrfs does the same thing
(check_direct_IO()). Is that a problem?