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fs/erofs/fileio: call erofs_onlinefolio_split() after bio_add_folio()

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commit bbfe756dc3062c1e934f06e5ba39c239aa953b92 upstream.

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 ce63cb62d7 ("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: ce63cb62d7 ("erofs: support unencoded inodes for fileio")
Fixes: 9f74ae8c9ac9 ("erofs: shorten bvecs[] for file-backed mounts")
Cc: stable@vger.kernel.org
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Reviewed-by: Gao Xiang <xiang@kernel.org>
Tested-by: Hongbo Li <lihongbo22@huawei.com>
Link: https://lore.kernel.org/r/20250428230933.3422273-1-max.kellermann@ionos.com
Signed-off-by: Gao Xiang <xiang@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Max Kellermann
2025-04-29 01:09:33 +02:00
committed by Greg Kroah-Hartman
parent b32411f045
commit 61e0fc3312
1 changed files with 2 additions and 2 deletions
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4
fs/erofs/fileio.c
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@@ -150,10 +150,10 @@ io_retry:
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;