Re: [PATCH RFC v3 1/4] KVM: guest_memfd: Track amount of memory allocated on inode

[David Hildenbrand (Arm)]

On 3/9/26 10:53, Ackerley Tng wrote:
> The guest memfd currently does not update the inode's i_blocks and i_bytes
> count when memory is allocated or freed. Hence, st_blocks returned from
> fstat() is always 0.
> 
> Introduce byte accounting for guest memfd inodes.  When a new folio is
> added to the filemap, add the folio's size.  Use the .invalidate_folio()
> callback to subtract the folio's size from inode fields when folios are
> truncated and removed from the filemap.
> 
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> ---
>  virt/kvm/guest_memfd.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 462c5c5cb602a..77219551056a7 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -136,6 +136,9 @@ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index)
>  					 mapping_gfp_mask(inode->i_mapping), policy);
>  	mpol_cond_put(policy);
>  
> +	if (!IS_ERR(folio))
> +		inode_add_bytes(inode, folio_size(folio));
> +

Can't we have two concurrent calls to __filemap_get_folio_mpol(), and we
don't really know whether our call allocated the folio or simply found
one (the other caller allocated) in the pagecache?

-- 
Cheers,

David