Clarifying what shmem pages are

I'm digging into the kernel VM and trying to understand what exactly shmem pages are. I'm trying to understand what the "Shmem" line in /proc/meminfo is signifying. I'm confused because I see notes saying shmem pages are in the page cache but I thought they aren't file-backed memory. In digging some more it seems shmem pages are backed by a virtual filesystem, shmemfs. From my understanding NR_FILE_PAGES indicates the number of file-backed pages and they are always in the page-cache. When I look into mm/shmem.c it seems NR_SHMEM_PAGES and NR_FILE_PAGES are always incremented and decremented together so I think maybe shmem pages are treated as file-backed but clarifying this would be great.