Re: [PATCH 1/3] mm: Move arch_do_swap_page() call to before swap_free()

David Hildenbrand
2023-05-19T09:22:27Z

>> 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.

--
Thanks,

David / dhildenb