Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.32 as permitted sender) client-ip=23.128.96.32;
From:   Ryan Roberts <ryan.roberts@arm.com>
To:     Andrew Morton <akpm@linux-foundation.org>,
        David Hildenbrand <david@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Huang Ying <ying.huang@intel.com>,
        Gao Xiang <xiang@kernel.org>, Yu Zhao <yuzhao@google.com>,
        Yang Shi <shy828301@gmail.com>, Michal Hocko <mhocko@suse.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>
Cc:     Ryan Roberts <ryan.roberts@arm.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: [PATCH v2 0/2] Swap-out small-sized THP without splitting
Date:   Tue, 17 Oct 2023 17:13:00 +0100
Message-Id: <20231017161302.2518826-1-ryan.roberts@arm.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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).


Thanks,
Ryan


[1] https://lore.kernel.org/linux-mm/20231010142111.3997780-1-ryan.roberts@arm.com/
[2] https://lore.kernel.org/linux-mm/15a52c3d-9584-449b-8228-1335e0753b04@arm.com/


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(-)

--
2.25.1