[PATCH RFC v2 4/6] KVM: guest_memfd: Track amount of memory allocated on inode

[Ackerley Tng <ackerleytng@google.com>]

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.  Conversely, when folios are
truncated and removed from the mapping, deduct the folio's size.

With this change, stat.st_blocks for a guest_memfd will correctly report
the number of 512-byte blocks allocated to the file, consistent with other
memory-based filesystems like tmpfs.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 virt/kvm/guest_memfd.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 2488d7b8f2b0d..b31e6612d16a8 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -137,6 +137,8 @@ static struct folio *__kvm_gmem_get_folio(struct inode *inode, pgoff_t index)
 		return ERR_PTR(ret);
 	}
 
+	inode_add_bytes(inode, folio_size(folio));
+
 	return folio;
 }
 
@@ -553,10 +555,16 @@ static void kvm_gmem_free_folio(struct folio *folio)
 }
 #endif
 
+static void kvm_gmem_unaccount_folio(struct folio *folio)
+{
+	__inode_sub_bytes(folio_inode(folio), folio_size(folio));
+}
+
 static const struct address_space_operations kvm_gmem_aops = {
 	.dirty_folio = noop_dirty_folio,
 	.migrate_folio	= kvm_gmem_migrate_folio,
 	.error_remove_folio = kvm_gmem_error_folio,
+	.unaccount_folio = kvm_gmem_unaccount_folio,
 #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_INVALIDATE
 	.free_folio = kvm_gmem_free_folio,
 #endif

-- 
2.53.0.414.gf7e9f6c205-goog