Re: [RFC V3 PATCH] arm64: mm: swap: save and restore mte tags for large folios

[Barry Song <21cnbao@gmail.com>]

On Thu, Dec 7, 2023 at 11:04 PM David Hildenbrand <david@redhat.com> wrote:
>
> >>
> >>> 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.
>

Fair enough. If we define "corner case" based on the percentage of those folios
which can get partial MTE tags set or get partial MTE tags invalidated, I agree
this is a corner case. I thought  that a corner case was a case which
could rarely
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.

right. I was thinking the original large folio was partially
swapped-in and forgot
the new allocated page was actually one folio with only one page :-)

Indeed, in that case, it is still restoring the MTE tag for the whole
folio with one
page.

>
> >
> > 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
>

Thanks
Barry