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

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

On 2/23/26 08:04, Ackerley Tng wrote:
> Hi,
> 
> Currently, guest_memfd doesn't update inode's i_blocks or i_bytes at
> all. Hence, st_blocks in the struct populated by a userspace fstat()
> call on a guest_memfd will always be 0. This patch series makes
> guest_memfd track the amount of memory allocated on an inode, which
> allows fstat() to accurately report that on requests from userspace.
> 
> The inode's i_blocks and i_bytes fields are updated when the folio is
> associated or disassociated from the guest_memfd inode, which are at
> allocation and truncation times respectively.
> 
> To update inode fields at truncation time, this series implements a
> custom truncation function for guest_memfd. An alternative would be to
> update truncate_inode_pages_range() to return the number of bytes
> truncated or add/use some hook.
> 
> Implementing a custom truncation function was chosen to provide
> flexibility for handling truncations in future when guest_memfd
> supports sources of pages other than the buddy allocator. This
> approach of a custom truncation function also aligns with shmem, which
> has a custom shmem_truncate_range().

Just wondered how shmem does it: it's through 
dquot_alloc_block_nodirty() / dquot_free_block_nodirty().

It's a shame we can't just use folio_free(). 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?

-- 
Cheers,

David