Messages in this thread | | | Date | Thu, 7 Dec 2023 11:03:58 +0100 | Subject | Re: [RFC V3 PATCH] arm64: mm: swap: save and restore mte tags for large folios | From | David Hildenbrand <> |
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>> >>> not per-folio? I'm also not sure what it buys us - instead of reading a per-page >>> flag we now have to read 128 bytes of tag for each page and check its zero. >> >> My point is, if that is the corner case, we might not care about that. > > Hi David,
Hi!
> my understanding is that this is NOT a corner. Alternatively, it is > really a common case.
If it happens with < 1% of all large folios on swapout/swapin, it's not the common case. Even if some scenarios you point out below can and will happen.
> > 1. a large folio can be partially unmapped when it is in swapche and > after it is swapped out > in all cases, its tags can be partially invalidated. I don't think > this is a corner case, as long > as userspaces are still working at the granularity of basepages, this > is always going to > happen. For example, userspace libc such as jemalloc can identify > PAGESIZE, and use > madvise(DONTNEED) to return memory to the kernel. Heap management is > still working > at the granularity of the basepage. > > 2. mprotect on a part of a large folio as Steven pointed out. > > 3.long term, we are working to swap-in large folios as a whole[1] just > like swapping out large > folios as a whole. for those ptes which are still contiguous swap > entries, i mean, which > are not unmapped by userspace after the large folios are swapped out > to swap devices, > we have a chance to swap in a whole large folio, we do have a chance > to restore tags > for the large folio without early-exit. but we still have a good > chance to fall back to base > page if we fail to allocate large folio, in this case, do_swap_page() > still works at the > granularity of basepage. and do_swap_page() will call swap_free(entry), tags of > > this particular page can be invalidated as a result.
I don't immediately see how that relates. You get a fresh small folio and simply load that tag from the internal datastructure. No messing with large folios required, because you don't have a large folio. So no considerations about large folio batch MTE tag restore apply.
> > 4. too many early-exit might be negative to performance. > > > So I am thinking that in the future, we need two helpers, > 1. void __arch_swap_restore(swp_entry_t entry, struct page *page); > this is always needed to support page-level tag restore. > > 2. void arch_swap_restore(swp_entry_t entry, struct folio *folio); > this can be a helper when we are able to swap in a whole folio. two > conditions must be met > (a). PTEs entries are still contiguous swap entries just as when large > folios were swapped > out. > (b). we succeed in the allocation of a large folio in do_swap_page. > > For this moment, we only need 1; we will add 2 in swap-in large folio series. > > What do you think?
I agree that it's better to keep it simple for now.
-- Cheers,
David / dhildenb
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