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

[David Hildenbrand]

On 17.11.23 00:47, Barry Song wrote:
> On Thu, Nov 16, 2023 at 5:36 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > On 15.11.23 21:49, Barry Song wrote:
> > > On Wed, Nov 15, 2023 at 11:16 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > > > On 14.11.23 02:43, Barry Song wrote:
> > > > > This patch makes MTE tags saving and restoring support large folios,
> > > > > then we don't need to split them into base pages for swapping out
> > > > > on ARM64 SoCs with MTE.
> > > > >
> > > > > arch_prepare_to_swap() should take folio rather than page as parameter
> > > > > because we support THP swap-out as a whole.
> > > > >
> > > > > Meanwhile, arch_swap_restore() should use page parameter rather than
> > > > > folio as swap-in always works at the granularity of base pages right
> > > > > now.
> > > >
> > > > ... but then we always have order-0 folios and can pass a folio, or what
> > > > am I missing?
> > >
> > > Hi David,
> > > you missed the discussion here:
> > >
> > > https://lore.kernel.org/lkml/CAGsJ_4yXjex8txgEGt7+WMKp4uDQTn-fR06ijv4Ac68MkhjMDw@xxxxxxxxxxxxxx/
> > > https://lore.kernel.org/lkml/CAGsJ_4xmBAcApyK8NgVQeX_Znp5e8D4fbbhGguOkNzmh1Veocg@xxxxxxxxxxxxxx/
> >
> > Okay, so you want to handle the refault-from-swapcache case where you get a
> > large folio.
> >
> > I was mislead by your "folio as swap-in always works at the granularity of
> > base pages right now" comment.
> >
> > What you actually wanted to say is "While we always swap in small folios, we
> > might refault large folios from the swapcache, and we only want to restore
> > the tags for the page of the large folio we are faulting on."
> >
> > But, I do if we can't simply restore the tags for the whole thing at once
> > at make the interface page-free?
> >
> > Let me elaborate:
> >
> > IIRC, if we have a large folio in the swapcache, the swap entries/offset are
> > contiguous. If you know you are faulting on page[1] of the folio with a
> > given swap offset, you can calculate the swap offset for page[0] simply by
> > subtracting from the offset.
> >
> > See page_swap_entry() on how we perform this calculation.
> >
> >
> > So you can simply pass the large folio and the swap entry corresponding
> > to the first page of the large folio, and restore all tags at once.
> >
> > So the interface would be
> >
> > arch_prepare_to_swap(struct folio *folio);
> > void arch_swap_restore(struct page *folio, swp_entry_t start_entry);
> >
> > I'm sorry if that was also already discussed.
>
> This has been discussed. Steven, Ryan and I all don't think this is a good
> option. in case we have a large folio with 16 basepages, as do_swap_page
> can only map one base page for each page fault, that means we have
> to restore 16(tags we restore in each page fault) * 16(the times of page faults)
> for this large folio.

Can't you remember that it's already been restored? That seems like a 
reasonable thing to have.

For large folios we have plenty of page flags in tail pages available?

--
Cheers,

David / dhildenb