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

[Keith Busch]

From: Keith Busch <kbusch at 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 at kernel.org>
Reviewed-by: Christoph Hellwig <hch at 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