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

[Catalin Marinas <catalin.marinas@arm.com>]

On Sat, Mar 23, 2024 at 12:41:36AM +1300, Barry Song wrote:
> From: Barry Song <v-songbaohua@oppo.com>
> 
> 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.
> 
> arch_prepare_to_swap() should take folio rather than page as parameter
> because we support THP swap-out as a whole. It saves tags for all
> pages in a large folio.
> 
> As now we are restoring tags based-on folio, in arch_swap_restore(),
> we may increase some extra loops and early-exitings while refaulting
> a large folio which is still in swapcache in do_swap_page(). In case
> a large folio has nr pages, do_swap_page() will only set the PTE of
> the particular page which is causing the page fault.
> Thus do_swap_page() runs nr times, and each time, arch_swap_restore()
> will loop nr times for those subpages in the folio. So right now the
> algorithmic complexity becomes O(nr^2).
> 
> Once we support mapping large folios in do_swap_page(), extra loops
> and early-exitings will decrease while not being completely removed
> as a large folio might get partially tagged in corner cases such as,
> 1. a large folio in swapcache can be partially unmapped, thus, MTE
> tags for the unmapped pages will be invalidated;
> 2. users might use mprotect() to set MTEs on a part of a large folio.
> 
> arch_thp_swp_supported() is dropped since ARM64 MTE was the only one
> who needed it.
> 
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Kemeng Shi <shikemeng@huaweicloud.com>
> Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> Cc: Peter Collingbourne <pcc@google.com>
> Cc: Steven Price <steven.price@arm.com>
> Cc: Yosry Ahmed <yosryahmed@google.com>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Lorenzo Stoakes <lstoakes@gmail.com>
> Cc: "Mike Rapoport (IBM)" <rppt@kernel.org>
> Cc: Hugh Dickins <hughd@google.com>
> CC: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
> Cc: Rick Edgecombe <rick.p.edgecombe@intel.com>
> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> Reviewed-by: Steven Price <steven.price@arm.com>
> Acked-by: Chris Li <chrisl@kernel.org>

Acked-by: Catalin Marinas <catalin.marinas@arm.com>