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From: Ryan Roberts <>
To: Andrew Morton <>,
	David Hildenbrand <>,
	Matthew Wilcox <>,
	Huang Ying <>, Gao Xiang <>,
	Yu Zhao <>, Yang Shi <>,
	Michal Hocko <>,
	Kefeng Wang <>
Cc: Ryan Roberts <>,,
Subject: [PATCH v2 0/2] Swap-out small-sized THP without splitting
Date: Tue, 17 Oct 2023 17:13:00 +0100	[thread overview]
Message-ID: <> (raw)

Hi All,

This is v2 of a series to add support for swapping out small-sized THP without
needing to first split the large folio via __split_huge_page(). It closely
follows the approach already used by PMD-sized THP.

"Small-sized THP" is an upcoming feature that enables performance improvements
by allocating large folios for anonymous memory, where the large folio size is
smaller than the traditional PMD-size. See [2].

In some circumstances I've observed a performance regression (see patch 2 for
details), and this series is an attempt to fix the regression in advance of
merging small-sized THP support.

I've done what I thought was the smallest change possible, and as a result, this
approach is only employed when the swap is backed by a non-rotating block device
(just as PMD-sized THP is supported today). Discussion against the RFC concluded
that this is probably sufficient.

The series applies against mm-unstable (3fb06e6d0a6f)

Changes since v1 [1]

 - patch 1:
    - Use cluster_set_count() instead of cluster_set_count_flag() in
      swap_alloc_cluster() since we no longer have any flag to set. I was unable
      to kill cluster_set_count_flag() as proposed against v1 as other call
      sites depend explicitly setting flags to 0.
 - patch 2:
    - Moved large_next[] array into percpu_cluster to make it per-cpu
      (recommended by Huang, Ying).
    - large_next[] array is dynamically allocated because PMD_ORDER is not
      compile-time constant for powerpc (fixes build error).



Ryan Roberts (2):
  mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags
  mm: swap: Swap-out small-sized THP without splitting

 include/linux/swap.h |  16 +++---
 mm/huge_memory.c     |   3 --
 mm/swapfile.c        | 119 ++++++++++++++++++++++++-------------------
 mm/vmscan.c          |  10 ++--
 4 files changed, 78 insertions(+), 70 deletions(-)


             reply	other threads:[~2023-10-17 16:13 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-10-17 16:13 Ryan Roberts [this message]
2023-10-17 16:13 ` [PATCH v2 1/2] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags Ryan Roberts
2023-10-17 16:13 ` [PATCH v2 2/2] mm: swap: Swap-out small-sized THP without splitting Ryan Roberts
2023-10-18  6:55   ` Huang, Ying
2023-10-18 14:07     ` Ryan Roberts
2023-10-19  5:49       ` Huang, Ying
2023-10-19 14:25         ` Ryan Roberts
2023-10-20  3:12           ` Huang, Ying

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