[PATCH 1/1] arm64: mm: swap: support THP_SWAP on hardware with MTE

[Catalin Marinas]

On Wed, Mar 27, 2024 at 03:13:18PM +0000, Ryan Roberts wrote:
> On 27/03/2024 14:57, David Hildenbrand wrote:
> > On 27.03.24 15:53, Matthew Wilcox wrote:
> >> On Sat, Mar 23, 2024 at 12:41:36AM +1300, Barry Song wrote:
> >>> Commit d0637c505f8a1 ("arm64: enable THP_SWAP for arm64") brings up
> >>> THP_SWAP on ARM64, but it doesn't enable THP_SWP on hardware with
> >>> MTE as the MTE code works with the assumption tags save/restore is
> >>> always handling a folio with only one page.
> >>>
> >>> The limitation should be removed as more and more ARM64 SoCs have
> >>> this feature. Co-existence of MTE and THP_SWAP becomes more and
> >>> more important.
> >>>
> >>> This patch makes MTE tags saving support large folios, then we don't
> >>> need to split large folios into base pages for swapping out on ARM64
> >>> SoCs with MTE any more.
> >>
> >> Can we go further than this patch and only support PG_mte_tagged and
> >> PG_mte_lock on folio->flags instead of page->flags?  We're down to using
> > 
> > I think we discussed that already and what I learned is that it "gets a bit
> > complicated". But I'm hoping we can get that discussion started again.
> 
> The original conversation starts here:
> https://lore.kernel.org/linux-mm/fb34d312-1049-4932-8f2b-d7f33cfc297c@arm.com/
> 
> The issue is that you can have a large folio mapped to user space, and user
> space only wants to activate MTE for a portion of it. So at that point, you
> either have to deal with only part of it being tagged (as we do today with the
> per-page flag) or you have to split the folio.

It needs splitting since the PROT_MTE property ends up in the pte as a
memory attribute. So we can't have a THP mapping with PROT_MTE but only
specific pages tagged.

I had an attempt last year to only keep the PG_mte_tagged flag in the
head page but I recall folio_copy() got in the way since it was calling
copy_highpage() on individual pages and the arm64 code was not seeing
the head PG_mte_tagged. I think it can be worked around but I got
distracted and forgot about this.

-- 
Catalin