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

[Ackerley Tng <ackerleytng@google.com>]

Ackerley Tng <ackerleytng@google.com> writes:

> "David Hildenbrand (Arm)" <david@kernel.org> 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.
>>

I'm not sure which lock you were referring to, I hope it's not the
inode's i_lock? Why is calling the callback under lock frowned upon?

I found .remove_folio also had to be called from
delete_from_page_cache_batch() for it to work. Then I saw that both of
those functions already use filemap_unaccount_folio(), and after all,
like you said, guest_memfd will be using this callback for accounting,
so in RFC v2 [1] I used .unaccount_folio instead, and it is called under
the inode's i_lock from filemap_unaccount_folio().

[1] https://lore.kernel.org/all/20260225-gmem-st-blocks-v2-0-87d7098119a9@google.com/T/

>> 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