On Tue, Apr 29, 2025 at 01:09:33AM +0200, Max Kellermann wrote:
> 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>
Thanks for catching this! LGTM:
Reviewed-by: Gao Xiang <xi...@kernel.org>
Thanks,
Gao Xiang
