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

[David Hildenbrand]

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.

BUT, IIRC in the context of

commit cfeed8ffe55b37fa10286aaaa1369da00cb88440
Author: David Hildenbrand <david@xxxxxxxxxx>
Date:   Mon Aug 21 18:08:46 2023 +0200

    mm/swap: stop using page->private on tail pages for THP_SWAP
    
    Patch series "mm/swap: stop using page->private on tail pages for THP_SWAP
    + cleanups".
    
    This series stops using page->private on tail pages for THP_SWAP, replaces
    folio->private by folio->swap for swapcache folios, and starts using
    "new_folio" for tail pages that we are splitting to remove the usage of
    page->private for swapcache handling completely.

As long as the folio is in the swapcache, we even do have the proper
swp_entry_t start_entry available as folio_swap_entry(folio).

But now I am confused when we actually would have to pass
"swp_entry_t start_entry". We shouldn't if the folio is in the swapcache ...

--
Cheers,

David / dhildenb