Re: [RFC PATCH v1 00/10] guest_memfd: Track amount of memory allocated on inode

[Ackerley Tng]

"David Hildenbrand (Arm)" <david@xxxxxxxxxx> writes:

>
> [...snip...]
>
>>> Could we maybe have a
>>> different callback (when the mapping is still guaranteed to be around)
>>> from where we could update i_blocks on the freeing path?
>>
>> Do you mean that we should add a new callback to struct
>> address_space_operations?
>
> If that avoids having to implement truncation completely ourselves, that might be one
> option we could discuss, yes.
>
> Something like:
>
> diff --git a/Documentation/filesystems/vfs.rst b/Documentation/filesystems/vfs.rst
> index 7c753148af88..94f8bb81f017 100644
> --- a/Documentation/filesystems/vfs.rst
> +++ b/Documentation/filesystems/vfs.rst
> @@ -764,6 +764,7 @@ cache in your filesystem.  The following members are defined:
>                 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 (*remove_folio)(struct folio *folio);
>                 void (*free_folio)(struct folio *);
>                 ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
>                 int (*migrate_folio)(struct mapping *, struct folio *dst,
> @@ -922,6 +923,11 @@ cache in your filesystem.  The following members are defined:
>         its release_folio will need to ensure this.  Possibly it can
>         clear the uptodate flag if it cannot free private data yet.
>
> +``remove_folio``
> +       remove_folio is called just before the folio is removed from the
> +       page cache in order to allow the cleanup of properties (e.g.,
> +       accounting) that needs the address_space mapping.
> +
>  ``free_folio``
>         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.
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 8b3dd145b25e..f7f6930977a1 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -422,6 +422,7 @@ struct address_space_operations {
>         sector_t (*bmap)(struct address_space *, sector_t);
>         void (*invalidate_folio) (struct folio *, size_t offset, size_t len);
>         bool (*release_folio)(struct folio *, gfp_t);
> +       void (*remove_folio)(struct folio *folio);
>         void (*free_folio)(struct folio *folio);
>         ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
>         /*
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 6cd7974d4ada..5a810eaacab2 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -250,8 +250,14 @@ void filemap_free_folio(struct address_space *mapping, struct folio *folio)
>  void filemap_remove_folio(struct folio *folio)
>  {
>         struct address_space *mapping = folio->mapping;
> +       void (*remove_folio)(struct folio *);
>
>         BUG_ON(!folio_test_locked(folio));
> +
> +       remove_folio = mapping->a_ops->remove_folio;
> +       if (unlikely(remove_folio))
> +               remove_folio(folio);
> +
>         spin_lock(&mapping->host->i_lock);
>         xa_lock_irq(&mapping->i_pages);
>         __filemap_remove_folio(folio, NULL);
>

Thanks for this suggestion, I'll try this out and send another revision.

>
> Ideally we'd perform it under the lock just after clearing folio->mapping, but I guess that
> might be more controversial.
>
> For accounting you need the above might be good enough, but I am not sure for how many
> other use cases there might be.
>
> --
> Cheers,
>
> David