[PATCHv6 10/11] block: relax direct io memory alignment

Keith Busch
2022-06-10T20:04:22Z

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. User
space can discover the alignment requirements from the dma_alignment
queue attribute.

User space can specify any hardware compatible DMA offset for each
segment, but every segment length is still required to be a multiple of
the block size.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---
block/bio.c | 9 +++++++++
block/fops.c | 4 ++--
include/linux/blkdev.h | 5 +++++
3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 5618c6a4b3a3..551f1d12208b 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1220,7 +1220,16 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
BUILD_BUG_ON(PAGE_PTRS_PER_BVEC < 2);
pages += entries_left * (PAGE_PTRS_PER_BVEC - 1);

+ /*
+ * Each segment in the iov is required to be a block size multiple.
+ * However, we may not be able to get the entire segment if it spans
+ * more pages than bi_max_vecs allows, so we have to ALIGN_DOWN the
+ * result to ensure the bio's total size is correct. The remainder of
+ * the iov data will be picked up in the next bio iteration.
+ */
size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
+ if (size > 0)
+ size = ALIGN_DOWN(size, bdev_logical_block_size(bio->bi_bdev));
if (unlikely(size <= 0))
return size ? size : -EFAULT;

diff --git a/block/fops.c b/block/fops.c
index 9d32df6fc315..86d3cab9bf93 100644
--- a/block/fops.c
+++ b/block/fops.c
@@ -45,8 +45,8 @@ static unsigned int dio_bio_write_op(struct kiocb *iocb)
static bool blkdev_dio_unaligned(struct block_device *bdev, loff_t pos,
struct iov_iter *iter)
{
- return ((pos | iov_iter_alignment(iter)) &
- (bdev_logical_block_size(bdev) - 1));
+ return pos & (bdev_logical_block_size(bdev) - 1) ||
+ !bdev_iter_is_aligned(bdev, iter);
}

#define DIO_INLINE_BIO_VECS 4
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index fb5c177708d5..914c613d81da 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -425,6 +425,11 @@ struct request_queue {
unsigned long nr_requests; /* Max # of requests */

unsigned int dma_pad_mask;
+ /*
+ * Drivers that set dma_alignment to less than 511 must be prepared to
+ * handle individual bvec's that are not a multiple of a SECTOR_SIZE
+ * due to possible offsets.
+ */
unsigned int dma_alignment;

#ifdef CONFIG_BLK_INLINE_ENCRYPTION
--
2.30.2