linux-kernel - [PATCHv2 2/7] block: align the bio after building it

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Message-ID: <20250805141123.332298-3-kbusch@meta.com>
Date: Tue, 5 Aug 2025 07:11:18 -0700
From: Keith Busch <kbusch@...a.com>
To: <linux-block@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
CC: <snitzer@...nel.org>, <axboe@...nel.dk>, <dw@...idwei.uk>,
        <brauner@...nel.org>, Keith Busch <kbusch@...nel.org>
Subject: [PATCHv2 2/7] block: align the bio after building it

From: Keith Busch <kbusch@...nel.org>

Instead of ensuring each vector is block size aligned while constructing
the bio, just ensure the entire size is aligned after it's built. This
makes it more flexible to accepting device valid io vectors that would
otherwise get rejected by alignment checks.

Signed-off-by: Keith Busch <kbusch@...nel.org>
---
 block/bio.c | 60 +++++++++++++++++++++++++++++++++++------------------
 1 file changed, 40 insertions(+), 20 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 92c512e876c8d..9ecd546c5b077 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1227,13 +1227,6 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 	if (bio->bi_bdev && blk_queue_pci_p2pdma(bio->bi_bdev->bd_disk->queue))
 		extraction_flags |= ITER_ALLOW_P2PDMA;
 
-	/*
-	 * 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_extract_pages(iter, &pages,
 				      UINT_MAX - bio->bi_iter.bi_size,
 				      nr_pages, extraction_flags, &offset);
@@ -1241,18 +1234,6 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 		return size ? size : -EFAULT;
 
 	nr_pages = DIV_ROUND_UP(offset + size, PAGE_SIZE);
-
-	if (bio->bi_bdev) {
-		size_t trim = size & (bdev_logical_block_size(bio->bi_bdev) - 1);
-		iov_iter_revert(iter, trim);
-		size -= trim;
-	}
-
-	if (unlikely(!size)) {
-		ret = -EFAULT;
-		goto out;
-	}
-
 	for (left = size, i = 0; left > 0; left -= len, i += num_pages) {
 		struct page *page = pages[i];
 		struct folio *folio = page_folio(page);
@@ -1297,6 +1278,43 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 	return ret;
 }
 
+static inline void bio_revert(struct bio *bio, unsigned int nbytes)
+{
+	bio->bi_iter.bi_size -= nbytes;
+
+	while (nbytes) {
+		struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
+
+		if (nbytes < bv->bv_len) {
+			bv->bv_len -= nbytes;
+			return;
+		}
+
+		bio_release_page(bio, bv->bv_page);
+		bio->bi_vcnt--;
+		nbytes -= bv->bv_len;
+       }
+}
+
+static int bio_align_to_lbs(struct bio *bio, struct iov_iter *iter)
+{
+	struct block_device *bdev = bio->bi_bdev;
+	size_t nbytes;
+
+	if (!bdev)
+		return 0;
+
+	nbytes = bio->bi_iter.bi_size & (bdev_logical_block_size(bdev) - 1);
+	if (!nbytes)
+		return 0;
+
+	bio_revert(bio, nbytes);
+	iov_iter_revert(iter, nbytes);
+	if (!bio->bi_iter.bi_size)
+		return -EFAULT;
+	return 0;
+}
+
 /**
  * bio_iov_iter_get_pages - add user or kernel pages to a bio
  * @bio: bio to add pages to
@@ -1336,7 +1354,9 @@ int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 		ret = __bio_iov_iter_get_pages(bio, iter);
 	} while (!ret && iov_iter_count(iter) && !bio_full(bio, 0));
 
-	return bio->bi_vcnt ? 0 : ret;
+	if (bio->bi_vcnt)
+		return bio_align_to_lbs(bio, iter);
+	return ret;
 }
 EXPORT_SYMBOL_GPL(bio_iov_iter_get_pages);
 
-- 
2.47.3