If bio_add_folio() fails (because it is full),
erofs_fileio_scan_folio() needs to submit the I/O request via
erofs_fileio_rq_submit() and allocate a new I/O request with an empty
`struct bio`. Then it retries the bio_add_folio() call.
However, at this point, erofs_onlinefolio_split() has already been
called which increments `folio->private`; the retry will call
erofs_onlinefolio_split() again, but there will never be a matching
erofs_onlinefolio_end() call. This leaves the folio locked forever
and all waiters will be stuck in folio_wait_bit_common().
This bug has been added by commit ce63cb62d794 ("erofs: support
unencoded inodes for fileio"), but was practically unreachable because
there was room for 256 folios in the `struct bio` - until commit
9f74ae8c9ac9 ("erofs: shorten bvecs[] for file-backed mounts") which
reduced the array capacity to 16 folios.
It was now trivial to trigger the bug by manually invoking readahead
from userspace, e.g.:
posix_fadvise(fd, 0, st.st_size, POSIX_FADV_WILLNEED);
This should be fixed by invoking erofs_onlinefolio_split() only after
bio_add_folio() has succeeded. This is safe: asynchronous completions
invoking erofs_onlinefolio_end() will not unlock the folio because
erofs_fileio_scan_folio() is still holding a reference to be released
by erofs_onlinefolio_end() at the end.
Fixes: ce63cb62d794 ("erofs: support unencoded inodes for fileio")
Fixes: 9f74ae8c9ac9 ("erofs: shorten bvecs[] for file-backed mounts")
Cc: sta...@vger.kernel.org
Signed-off-by: Max Kellermann <max.kellerm...@ionos.com>
---
fs/erofs/fileio.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/fs/erofs/fileio.c b/fs/erofs/fileio.c
index 4fa0a0121288..60c7cc4c105c 100644
--- a/fs/erofs/fileio.c
+++ b/fs/erofs/fileio.c
@@ -150,10 +150,10 @@ static int erofs_fileio_scan_folio(struct erofs_fileio
*io, struct folio *folio)
io->rq->bio.bi_iter.bi_sector = io->dev.m_pa >>
9;
attached = 0;
}
- if (!attached++)
- erofs_onlinefolio_split(folio);
if (!bio_add_folio(&io->rq->bio, folio, len, cur))
goto io_retry;
+ if (!attached++)
+ erofs_onlinefolio_split(folio);
io->dev.m_pa += len;
}
cur += len;
--
2.47.2
