On 07/22, Eric Biggers wrote:
> On Fri, Jul 22, 2022 at 08:43:55AM -0600, Keith Busch wrote:
> > 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 <kbu...@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 <kbu...@kernel.org>
> > > > Reviewed-by: Christoph Hellwig <h...@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().
>
> No. It's a bit subtle, so read the code and what I'm saying carefully. f2fs
> only supports 4K aligned DIO and normally falls back to buffered I/O; however,
> for DIO that is *very* misaligned (not even LBS aligned) it returns EINVAL
> instead. And it relies on __iomap_dio_rw() returning that EINVAL.
>
> Relying on __iomap_dio_rw() in that way is definitely a bad design on f2fs's
> part (and I messed that up when switching f2fs from fs/direct-io.c to iomap).
> The obvious fix is to just have f2fs do the LBS alignment check itself.
>
> But I think that f2fs shouldn't have different behavior for different levels
> of
> misalignment in the first place, so I was wondering if anyone had any thoughts
> on which behavior (EINVAL or fallback to buffered I/O) should be standardized
> on
> in all cases, at least for f2fs. There was some discussion about this sort of
> thing for ext4 several years ago on the thread
> https://lore.kernel.org/linux-ext4/1461472078-20104-1-git-send-email-ty...@mit.edu/T/#u,
> but it didn't really reach a conclusion. I'm wondering if the f2fs
> maintainers
> have any thoughts about why the f2fs behavior is as it is. I.e. is it just
> accidental, or are there specific reasons...
If there's a generic way to deal with this, I have no objection to
follow it. Initially, I remember I was trying to match the ext4 rule,
but at some point, I lost the track.
>
> - Eric
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