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filemap: Handle sibling entries in filemap_get_read_batch() 

commit cb995f4eeb upstream.

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: cbd59c48ae ("mm/filemap: use head pages in generic_file_buffered_read")
Cc: stable@vger.kernel.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Matthew Wilcox (Oracle) 2022-06-17 20:00:17 -04:00 committed by Greg Kroah-Hartman
parent 8a79182e7a
commit a66f131d30
1 changed files with 2 additions and 0 deletions
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2
mm/filemap.c
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@ -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;