[PATCH] block: free copied pages when blk_rq_map_kern() fails

[PATCH] block: free copied pages when blk_rq_map_kern() fails

[Jackie Liu]

From: Jackie Liu <liuyun01@kylinos.cn>

bio_copy_kern() allocates pages that are normally freed by the bio
completion callback. If blk_rq_append_bio() rejects the bio, however,
blk_rq_map_kern() only drops the bio reference. Since bio_put() does not
free pages referenced by the bio vectors, those pages leak.

This can happen when the bio exceeds the queue segment constraints or
when a later mapping cannot be merged into a request built by earlier
calls. Track whether the buffer was copied and free those pages before
dropping the rejected bio.

Fixes: 3a5a39276d2a ("block: allow blk_rq_map_kern to append to requests")
Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
---
 block/blk-map.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/block/blk-map.c b/block/blk-map.c
index 768549f19f97..d1d6bbe0ecf1 100644
--- a/block/blk-map.c
+++ b/block/blk-map.c
@@ -653,6 +653,7 @@ int blk_rq_map_kern(struct request *rq, void *kbuf, unsigned int len,
 		gfp_t gfp_mask)
 {
 	unsigned long addr = (unsigned long) kbuf;
+	bool do_copy;
 	struct bio *bio;
 	int ret;
 
@@ -661,7 +662,8 @@ int blk_rq_map_kern(struct request *rq, void *kbuf, unsigned int len,
 	if (!len || !kbuf)
 		return -EINVAL;
 
-	if (!blk_rq_aligned(rq->q, addr, len) || object_is_on_stack(kbuf))
+	do_copy = !blk_rq_aligned(rq->q, addr, len) || object_is_on_stack(kbuf);
+	if (do_copy)
 		bio = bio_copy_kern(rq, kbuf, len, gfp_mask);
 	else
 		bio = bio_map_kern(rq, kbuf, len, gfp_mask);
@@ -670,8 +672,11 @@ int blk_rq_map_kern(struct request *rq, void *kbuf, unsigned int len,
 		return PTR_ERR(bio);
 
 	ret = blk_rq_append_bio(rq, bio);
-	if (unlikely(ret))
+	if (unlikely(ret)) {
+		if (do_copy)
+			bio_free_pages(bio);
 		blk_mq_map_bio_put(bio);
+	}
 	return ret;
 }
 EXPORT_SYMBOL(blk_rq_map_kern);
-- 
2.54.0

Re: [PATCH] block: free copied pages when blk_rq_map_kern() fails

[Christoph Hellwig]

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH] block: free copied pages when blk_rq_map_kern() fails

[Jens Axboe]

On Wed, 15 Jul 2026 15:35:18 +0800, Jackie Liu wrote:
> bio_copy_kern() allocates pages that are normally freed by the bio
> completion callback. If blk_rq_append_bio() rejects the bio, however,
> blk_rq_map_kern() only drops the bio reference. Since bio_put() does not
> free pages referenced by the bio vectors, those pages leak.
> 
> This can happen when the bio exceeds the queue segment constraints or
> when a later mapping cannot be merged into a request built by earlier
> calls. Track whether the buffer was copied and free those pages before
> dropping the rejected bio.
> 
> [...]

Applied, thanks!

[1/1] block: free copied pages when blk_rq_map_kern() fails
      commit: bd2df8dc72201f626d66ca97f2afc8b7f2740713

Best regards,
-- 
Jens Axboe