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block: revert the iov_iter after a short copy in bio_iov_iter_bounce_write()

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For the incoming IOMAP_DIO_BOUNCE flag usage inside btrfs, it's pretty
easy to hit short copy inside bio_iov_iter_bounce_write().

This is because btrfs has disabled page fault to avoid certain deadlock
during direct writes, and instead btrfs manually fault in the pages then
retry.

And inside bio_iov_iter_bounce_write(), if we hit a short write, we
didn't revert the iov_iter, which can cause problems like unexpected
garbage for the next retry.

Revert the iov_iter after a short copy.

One thing to note is that, the folio is allocated then immediately
queued into the bio, so the proper revert size should be
(bi_size - this_len + copied).

Fixes: 8dd5e7c75d ("block: add helpers to bounce buffer an iov_iter into bios")
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/c400989f227343b134110773d5acaaacf7024574.1781597506.git.wqu@suse.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Qu Wenruo 2026-06-16 17:42:35 +09:30 committed by Jens Axboe
parent fad156c2af
commit b68d4979c8
1 changed files with 14 additions and 3 deletions
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17
block/bio.c
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@ -1321,6 +1321,7 @@ static int bio_iov_iter_bounce_write(struct bio *bio, struct iov_iter *iter,
do {
size_t this_len = min(total_len, SZ_1M);
size_t copied;
struct folio *folio;
if (this_len > minsize * 2)
@ -1334,12 +1335,22 @@ static int bio_iov_iter_bounce_write(struct bio *bio, struct iov_iter *iter,
break;
bio_add_folio_nofail(bio, folio, this_len, 0);
if (copy_from_iter(folio_address(folio), this_len, iter) !=
this_len) {
copied = copy_from_iter(folio_address(folio), this_len, iter);
if (copied < this_len) {
/*
* Need to revert the iov iter for all bytes we have
* copied.
*
* However the bio size differs from the real copied
* bytes as @this_len is queued but only advanced
* less than that.
* Need to compensate that for the revert.
*/
iov_iter_revert(iter, bio->bi_iter.bi_size - this_len +
copied);
bio_free_folios(bio);
return -EFAULT;
}
total_len -= this_len;
} while (total_len && bio->bi_vcnt < bio->bi_max_vecs);