Re: [PATCH 1/2] ceph: free page array when ceph_submit_write() fails

[Ilya Dryomov]

On Mon, Jan 26, 2026 at 3:27 AM Sam Edwards <cfsworks@xxxxxxxxx> wrote:
>
> If `locked_pages` is zero, the page array must not be allocated:
> ceph_process_folio_batch() uses `locked_pages` to decide when to
> allocate `pages`, and redundant allocations trigger
> ceph_allocate_page_array()'s BUG_ON(), resulting in a worker oops (and
> writeback stall) or even a kernel panic. Consequently, the main loop in
> ceph_writepages_start() assumes that the lifetime of `pages` is confined
> to a single iteration.
>
> The ceph_submit_write() function claims ownership of the page array on
> success (it is later freed when the write concludes). But failures only
> redirty/unlock the pages and fail to free the array, making the failure
> case in ceph_submit_write() fatal.
>
> Free the page array (and reset locked_pages) in ceph_submit_write()'s
> error-handling 'if' block so that the caller's invariant (that the array
> does not remain in ceph_wbc) is maintained unconditionally, making
> failures in ceph_submit_write() recoverable as originally intended.
>
> Fixes: 1551ec61dc55 ("ceph: introduce ceph_submit_write() method")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sam Edwards <CFSworks@xxxxxxxxx>
> ---
>  fs/ceph/addr.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
> index 63b75d214210..c3e0b5b429ea 100644
> --- a/fs/ceph/addr.c
> +++ b/fs/ceph/addr.c
> @@ -1470,6 +1470,14 @@ int ceph_submit_write(struct address_space *mapping,
>                         unlock_page(page);
>                 }
>
> +               if (ceph_wbc->from_pool) {
> +                       mempool_free(ceph_wbc->pages, ceph_wb_pagevec_pool);
> +                       ceph_wbc->from_pool = false;
> +               } else
> +                       kfree(ceph_wbc->pages);
> +               ceph_wbc->pages = NULL;
> +               ceph_wbc->locked_pages = 0;

Hi Sam,

While I don't see anything wrong with the patch per se, I can't help
but question the existence of this entire branch along with the meaning
of the error.

ceph_writepages_start() is the only caller of ceph_submit_write() and
it already calls ceph_inc_osd_stopping_blocker() at the top where the
error can be handled naturally -- nothing needs to be unlocked or freed
at that point.  Since mdsc->stopping_blockers is just a counter, all
calls made by ceph_submit_write() invocations in a loop would be
"contained" within that ceph_writepages_start() call.  The only benefit
achieved is potentially faster response to the MDS client moving to
CEPH_MDSC_STOPPING_FLUSHING state, but it's rather dubious because
sneaking in/having to wait for some more OSD requests isn't really the
end of the world.

Rather than patching the error path, I wonder if instead of calling
ceph_inc_osd_stopping_blocker() the counter could just be incremented
unconditionally, with the check for CEPH_MDSC_STOPPING_FLUSHING bypassed
there?  This could be wrapped into a new helper that could also assert
that the counter is already elevated before the increment.

Thanks,

                Ilya