Re: [PATCH 2/3] filemap: Handle sibling entries in filemap_get_read_batch()

[Brian Foster <bfoster@redhat.com>]

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@fromorbit.com>
> Reported-by: Brian Foster <bfoster@redhat.com>
> Debugged-by: Brian Foster <bfoster@redhat.com>
> Tested-by: Brian Foster <bfoster@redhat.com>
> Fixes: cbd59c48ae2b ("mm/filemap: use head pages in generic_file_buffered_read")
> Cc: stable@vger.kernel.org
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---

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@redhat.com>

>  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
>