linux-kernel - Re: [PATCH v4 0/4] mm: batched unmap lazyfree large folios during reclamation

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Message-ID: <38c757b6-4f60-40e3-9b77-69c9b4ccc004@lucifer.local>
Date: Wed, 25 Jun 2025 14:49:51 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
        baolin.wang@...ux.alibaba.com, chrisl@...nel.org, david@...hat.com,
        ioworker0@...il.com, kasong@...cent.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org, ryan.roberts@....com,
        v-songbaohua@...o.com, x86@...nel.org, ying.huang@...el.com,
        zhengtangquan@...o.com, Rik van Riel <riel@...riel.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Harry Yoo <harry.yoo@...cle.com>
Subject: Re: [PATCH v4 0/4] mm: batched unmap lazyfree large folios during
 reclamation

+cc Rik, Liam, Vlastimil, Harry

This touches rmap so please cc rmap reviewers. On future revisions of this
series!

On Fri, Feb 14, 2025 at 10:30:11PM +1300, Barry Song wrote:
> This patchset resolves the issue by marking only genuinely dirty folios
> as swap-backed, as suggested by David, and transitioning to batched
> unmapping of entire folios in try_to_unmap_one(). Consequently, the
> deferred_split count drops to zero, and memory reclamation performance
> improves significantly — reclaiming 64KiB lazyfree large folios is now
> 2.5x faster(The specific data is embedded in the changelog of patch
> 3/4).

Nice!

>
> By the way, while the patchset is primarily aimed at PTE-mapped large
> folios, Baolin and Lance also found that try_to_unmap_one() handles
> lazyfree redirtied PMD-mapped large folios inefficiently — it splits
> the PMD into PTEs and iterates over them. This patchset removes the
> unnecessary splitting, enabling us to skip redirtied PMD-mapped large
> folios 3.5X faster during memory reclamation. (The specific data can
> be found in the changelog of patch 4/4).

Also nice!