On Wed, May 17, 2023 at 1:30 AM David Hildenbrand david@redhat.com wrote:
Would the idea be to fail swap_readpage() on the one that comes last, simply retrying to lookup the page?
The idea would be that T2's arch_swap_readpage() could potentially not find tags if it ran after swap_free(), so T2 would produce a page without restored tags. But that wouldn't matter, because T1 reaching swap_free() means that T2 will follow the goto at [1] after waiting for T1 to unlock at [2], and T2's page will be discarded.
Ah, right.
This might be a naive question, but how does MTE play along with shared anonymous pages?
It should work fine. shmem_writepage() calls swap_writepage() which calls arch_prepare_to_swap() to write the tags. And shmem_swapin_folio() has a call to arch_swap_restore() to restore them.
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).
Peter