On Sat, Jun 25, 2022 at 04:44:59PM +0200, gregkh@xxxxxxxxxxxxxxxxxxx wrote:
>
> The patch below does not apply to the 5.15-stable tree.
> If someone wants it applied there, or to any other stable or longterm
> tree, then please email the backport, including the original git commit
> id to <stable@xxxxxxxxxxxxxxx>.
Compile-tested only, but it's fairly straightforward (if you understand
how the code mutated over time)
diff --git a/mm/filemap.c b/mm/filemap.c
index 00e391e75880..9cef95427575 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2341,6 +2341,8 @@ static void filemap_get_read_batch(struct address_space *mapping,
continue;
if (xas.xa_index > max || xa_is_value(head))
break;
+ if (xa_is_sibling(head))
+ break;
if (!page_cache_get_speculative(head))
goto retry;
> thanks,
>
> greg k-h
>
> ------------------ original commit in Linus's tree ------------------
>
> >From cb995f4eeba9d268fd4b56c2423ad6c1d1ea1b82 Mon Sep 17 00:00:00 2001
> From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> Date: Fri, 17 Jun 2022 20:00:17 -0400
> Subject: [PATCH] filemap: Handle sibling entries in filemap_get_read_batch()
>
> 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>
> Reviewed-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>
>
> 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;
>
>
