2022-06-19 15:16:02

by Matthew Wilcox

Subject: [PATCH 2/3] 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 <[email protected]>
Reported-by: Brian Foster <[email protected]>
Debugged-by: Brian Foster <[email protected]>
Tested-by: Brian Foster <[email protected]>
Fixes: cbd59c48ae2b ("mm/filemap: use head pages in generic_file_buffered_read")
Cc: [email protected]
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
---
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

2022-06-20 10:23:02

by Brian Foster

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

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 <[email protected]>
> Reported-by: Brian Foster <[email protected]>
> Debugged-by: Brian Foster <[email protected]>
> Tested-by: Brian Foster <[email protected]>
> Fixes: cbd59c48ae2b ("mm/filemap: use head pages in generic_file_buffered_read")
> Cc: [email protected]
> Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
> ---

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 <[email protected]>

> 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
>