Re: [PATCH v6 01/11] filemap: Pass address_space mapping to ->free_folio()

[Hugh Dickins <hughd@google.com>]

On Fri, 12 Sep 2025, Roy, Patrick wrote:

> From: Elliot Berman <quic_eberman@quicinc.com>
> 
> When guest_memfd removes memory from the host kernel's direct map,
> direct map entries must be restored before the memory is freed again. To
> do so, ->free_folio() needs to know whether a gmem folio was direct map
> removed in the first place though. While possible to keep track of this
> information on each individual folio (e.g. via page flags), direct map
> removal is an all-or-nothing property of the entire guest_memfd, so it
> is less error prone to just check the flag stored in the gmem inode's
> private data.  However, by the time ->free_folio() is called,
> folio->mapping might be cleared. To still allow access to the address
> space from which the folio was just removed, pass it in as an additional
> argument to ->free_folio, as the mapping is well-known to all callers.
> 
> Link: https://lore.kernel.org/all/15f665b4-2d33-41ca-ac50-fafe24ade32f@redhat.com/
> Suggested-by: David Hildenbrand <david@redhat.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
> [patrick: rewrite shortlog for new usecase]
> Signed-off-by: Patrick Roy <roypat@amazon.co.uk>
> ---
>  Documentation/filesystems/locking.rst |  2 +-
>  fs/nfs/dir.c                          | 11 ++++++-----
>  fs/orangefs/inode.c                   |  3 ++-
>  include/linux/fs.h                    |  2 +-
>  mm/filemap.c                          |  9 +++++----
>  mm/secretmem.c                        |  3 ++-
>  mm/vmscan.c                           |  4 ++--
>  virt/kvm/guest_memfd.c                |  3 ++-
>  8 files changed, 21 insertions(+), 16 deletions(-)
> 
> diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
> index aa287ccdac2f..74c97287ec40 100644
> --- a/Documentation/filesystems/locking.rst
> +++ b/Documentation/filesystems/locking.rst
> @@ -262,7 +262,7 @@ prototypes::
>  	sector_t (*bmap)(struct address_space *, sector_t);
>  	void (*invalidate_folio) (struct folio *, size_t start, size_t len);
>  	bool (*release_folio)(struct folio *, gfp_t);
> -	void (*free_folio)(struct folio *);
> +	void (*free_folio)(struct address_space *, struct folio *);
>  	int (*direct_IO)(struct kiocb *, struct iov_iter *iter);
>  	int (*migrate_folio)(struct address_space *, struct folio *dst,
>  			struct folio *src, enum migrate_mode);

Beware, that is against the intent of free_folio().

Since its 2.6.37 origin in 6072d13c4293 ("Call the filesystem back
whenever a page is removed from the page cache"), freepage() or
free_folio() has intentionally NOT taken a struct address_space *mapping,
because that structure may already be freed by the time free_folio() is
called, if the last folio holding it has now been freed.

Maybe something has changed since then, or maybe it happens to be safe
just in the context in which you want to use it; but it is against the
principle of free_folio().  (Maybe an rcu_read_lock() could be added
in __remove_mapping() to make it safe nowadays? maybe not welcome.)

See Documentation/filesystems/vfs.rst:
free_folio is called once the folio is no longer visible in the
page cache in order to allow the cleanup of any private data.
Since it may be called by the memory reclaimer, it should not
assume that the original address_space mapping still exists, and
it should not block.

Hugh