linux-kernel - [PATCH v3 0/3] Optimizations for khugepaged

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Message-Id: <20250722150559.96465-1-dev.jain@arm.com>
Date: Tue, 22 Jul 2025 20:35:56 +0530
From: Dev Jain <dev.jain@....com>
To: akpm@...ux-foundation.org,
	david@...hat.com
Cc: ziy@...dia.com,
	baolin.wang@...ux.alibaba.com,
	lorenzo.stoakes@...cle.com,
	Liam.Howlett@...cle.com,
	npache@...hat.com,
	ryan.roberts@....com,
	baohua@...nel.org,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Dev Jain <dev.jain@....com>
Subject: [PATCH v3 0/3] Optimizations for khugepaged

If the underlying folio mapped by the ptes is large, we can process those
ptes in a batch using folio_pte_batch().

For arm64 specifically, this results in a 16x reduction in the number of
ptep_get() calls, since on a contig block, ptep_get() on arm64 will iterate
through all 16 entries to collect a/d bits. Next, ptep_clear() will cause
a TLBI for every contig block in the range via contpte_try_unfold().
Instead, use clear_ptes() to only do the TLBI at the first and last
contig block of the range.

For split folios, there will be no pte batching; the batch size returned
by folio_pte_batch() will be 1. For pagetable split folios, the ptes will
still point to the same large folio; for arm64, this results in the
optimization described above, and for other arches, a minor improvement
is expected due to a reduction in the number of function calls and
batching atomic operations.

---
Rebased on today's mm-new.

v2->v3:
 - Drop patch 3 (was merged separately)
 - Add patch 1 (David)
 - Coding style change, drop mapped_folio (Lorenzo)

v1->v2:
 - Use for loop instead of do-while loop (Lorenzo)
 - Remove folio_test_large check since the subpage-check condition
   will imply that (Baolin)
 - Combine patch 1 and 2 into this series, add new patch 3

David Hildenbrand (1):
  mm: add get_and_clear_ptes() and clear_ptes()

Dev Jain (2):
  khugepaged: Optimize __collapse_huge_page_copy_succeeded() by PTE
    batching
  khugepaged: Optimize collapse_pte_mapped_thp() by PTE batching

 arch/arm64/mm/mmu.c     |  2 +-
 include/linux/pgtable.h |  6 +++++
 mm/khugepaged.c         | 57 +++++++++++++++++++++++++++--------------
 mm/mremap.c             |  2 +-
 mm/rmap.c               |  2 +-
 5 files changed, 47 insertions(+), 22 deletions(-)

-- 
2.30.2