Matthew Wilcox (Oracle) [off-list ref] wrote:

memcg information is only stored in the head page, so the memcg
subsystem needs to assure that all accesses are to the head page.
The first step is converting page_memcg() to folio_memcg().

The callers of page_memcg() and PageMemcgKmem() are not yet ready to be
converted to use folios, so retain them as wrappers around folio_memcg()
and folio_memcg_kmem().  They will be converted in a later patch set.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

Reviewed-by: David Howells <dhowells@redhat.com>