On Sun, Jun 19, 2022 at 04:11:42PM +0100, Matthew Wilcox (Oracle) wrote:
> If a read races with an invalidation followed by another read, it is
> possible for a folio to be replaced with a higher-order folio. If that
> happens, we'll see a sibling entry for the new folio in the next iteration
> of the loop. This manifests as a NULL pointer dereference while holding
> the RCU read lock.
>
> Handle this by simply returning. The next call will find the new folio
> and handle it correctly. The other ways of handling this rare race are
> more complex and it's just not worth it.
>
> Reported-by: Dave Chinner <david@xxxxxxxxxxxxx>
> Reported-by: Brian Foster <bfoster@xxxxxxxxxx>
> Debugged-by: Brian Foster <bfoster@xxxxxxxxxx>
> Tested-by: Brian Foster <bfoster@xxxxxxxxxx>
> Fixes: cbd59c48ae2b ("mm/filemap: use head pages in generic_file_buffered_read")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
This has survived my testing for several days now. I think I grok the
fix and analysis that lead to it, so FWIW:
Reviewed-by: Brian Foster <bfoster@xxxxxxxxxx>
> mm/filemap.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 577068868449..ffdfbc8b0e3c 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2385,6 +2385,8 @@ static void filemap_get_read_batch(struct address_space *mapping,
> continue;
> if (xas.xa_index > max || xa_is_value(folio))
> break;
> + if (xa_is_sibling(folio))
> + break;
> if (!folio_try_get_rcu(folio))
> goto retry;
>
> --
> 2.35.1
>
