Sorry, I meant actual anonymous memory pages, not shmem. Like, anonymous pages that are COW-shared due to fork() or KSM.
How does MTE, in general, interact with that? Assume one process ends up modifying the tags ... and the page is COW-shared with a different process that should not observe these tag modifications.
Tag modifications cause write faults if the page is read-only, so for COW shared pages we would end up copying the page in the usual way, which on arm64 would copy the tags as well via the copy_highpage hook (see arch/arm64/mm/copypage.c).
Oh, that makes sense, thanks for pointing that out!
... and I can spot that KSM also checks the tag when de-duplicating: pages_identical() ends up calling memcmp_pages(), which knows how to deal with tags.
Interestingly, calc_checksum() does not seem to care about tags. But that simply implies that pages with the same content have same checksum, independent of the tag. And pages_identical() is the single source of truth.