Re: [PATCH 6.18.y] mm: call ->free_folio() directly in folio_unmap_invalidate()

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Apr 20, 2026 at 03:53:43PM +0100, Matthew Wilcox (Oracle) wrote:
> We can only call filemap_free_folio() if we have a reference to (or hold a
> lock on) the mapping.  Otherwise, we've already removed the folio from the
> mapping so it no longer pins the mapping and the mapping can be removed,
> causing a use-after-free when accessing mapping->a_ops.
> 
> Follow the same pattern as __remove_mapping() and load the free_folio
> function pointer before dropping the lock on the mapping.  That lets us
> make filemap_free_folio() static as this was the only caller outside
> filemap.c.
> 
> Link: https://lore.kernel.org/20260413184314.3419945-1-willy@infradead.org
> Fixes: fb7d3bc41493 ("mm/filemap: drop streaming/uncached pages when writeback completes")
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Reported-by: Google Big Sleep <big-sleep-vuln-reports+bigsleep-501448199@google.com>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Jan Kara <jack@suse.cz>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> (cherry picked from commit 615d9bb2ccad42f9e21d837431e401db2e471195)
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  mm/filemap.c  | 3 ++-
>  mm/internal.h | 1 -
>  mm/truncate.c | 6 +++++-
>  3 files changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index d8d9c0f0beb6..76bbfa69aca0 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -233,7 +233,8 @@ void __filemap_remove_folio(struct folio *folio, void *shadow)
>  	page_cache_delete(mapping, folio, shadow);
>  }
>  
> -void filemap_free_folio(struct address_space *mapping, struct folio *folio)
> +static void filemap_free_folio(const struct address_space *mapping,
> +		struct folio *folio)
>  {
>  	void (*free_folio)(struct folio *);
>  	int refs = 1;
> diff --git a/mm/internal.h b/mm/internal.h
> index 9e0577413087..f046099d8eff 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -401,7 +401,6 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t *start,
>  		pgoff_t end, struct folio_batch *fbatch, pgoff_t *indices);
>  unsigned find_get_entries(struct address_space *mapping, pgoff_t *start,
>  		pgoff_t end, struct folio_batch *fbatch, pgoff_t *indices);
> -void filemap_free_folio(struct address_space *mapping, struct folio *folio);
>  int truncate_inode_folio(struct address_space *mapping, struct folio *folio);
>  bool truncate_inode_partial_folio(struct folio *folio, loff_t start,
>  		loff_t end);
> diff --git a/mm/truncate.c b/mm/truncate.c
> index fb5c20b57bd4..6bbe22ae3ab8 100644
> --- a/mm/truncate.c
> +++ b/mm/truncate.c
> @@ -574,6 +574,7 @@ EXPORT_SYMBOL(invalidate_mapping_pages);
>  static int invalidate_complete_folio2(struct address_space *mapping,
>  					struct folio *folio)
>  {
> +	void (*free_folio)(struct folio *);
>  	if (folio->mapping != mapping)
>  		return 0;
>  
> @@ -590,9 +591,12 @@ static int invalidate_complete_folio2(struct address_space *mapping,
>  	xa_unlock_irq(&mapping->i_pages);
>  	if (mapping_shrinkable(mapping))
>  		inode_add_lru(mapping->host);
> +	free_folio = mapping->a_ops->free_folio;

Wait, I see what's wrong, this function isn't even in 6.18.y, which is
why when I apply this it has fuzz and blows up.

So this isn't going to work at all here, did you send the wrong
backport?

thanks,

greg k-h