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

["Darrick J. Wong" <djwong@kernel.org>]

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))

How does this change intersect with "make statx() return DIO alignment
information" ?  Will the new STATX_DIOALIGN implementations have to be
adjusted to set stx_dio_mem_align = bdev_dma_alignment(...)?

I'm guessing the answer is yes, but I haven't seen any patches on the
list to do that, but more and more these days email behaves like a flood
of UDP traffic... :(

--D

> >  		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?
> 
> (Note: if you blame the above code, it was written by me.  But I was just
> preserving the existing behavior; I don't know the original motivation.)
> 
> - Eric