On Sat, Jun 25, 2022 at 04:44:59PM +0200, gregkh@linuxfoundation.org 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@vger.kernel.org.

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@infradead.org 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@fromorbit.com Reported-by: Brian Foster bfoster@redhat.com Debugged-by: Brian Foster bfoster@redhat.com Tested-by: Brian Foster bfoster@redhat.com Reviewed-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

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;