linux-kernel - Re: [PATCH v4 01/16] mm: thp: Batch-collapse PMD with set

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Date: Tue, 19 Dec 2023 08:18:17 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Ard Biesheuvel <ardb@...nel.org>, Marc Zyngier <maz@...nel.org>,
 Oliver Upton <oliver.upton@...ux.dev>, James Morse <james.morse@....com>,
 Suzuki K Poulose <suzuki.poulose@....com>, Zenghui Yu
 <yuzenghui@...wei.com>, Andrey Ryabinin <ryabinin.a.a@...il.com>,
 Alexander Potapenko <glider@...gle.com>,
 Andrey Konovalov <andreyknvl@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>,
 Vincenzo Frascino <vincenzo.frascino@....com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Anshuman Khandual <anshuman.khandual@....com>,
 Matthew Wilcox <willy@...radead.org>, Yu Zhao <yuzhao@...gle.com>,
 Mark Rutland <mark.rutland@....com>, Kefeng Wang
 <wangkefeng.wang@...wei.com>, John Hubbard <jhubbard@...dia.com>,
 Zi Yan <ziy@...dia.com>, Barry Song <21cnbao@...il.com>,
 Alistair Popple <apopple@...dia.com>, Yang Shi <shy828301@...il.com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 01/16] mm: thp: Batch-collapse PMD with set_ptes()

On 18/12/2023 17:40, David Hildenbrand wrote:
> On 18.12.23 11:50, Ryan Roberts wrote:
>> Refactor __split_huge_pmd_locked() so that a present PMD can be
>> collapsed to PTEs in a single batch using set_ptes(). It also provides a
>> future opportunity to batch-add the folio to the rmap using David's new
>> batched rmap APIs.
> 
> I'd drop that sentence and rather just say "In the future, we might get rid of
> the remaining manual loop by using rmap batching.".

OK fair enough. Will fix for next version.

> 
>>
>> This should improve performance a little bit, but the real motivation is
>> to remove the need for the arm64 backend to have to fold the contpte
>> entries. Instead, since the ptes are set as a batch, the contpte blocks
>> can be initially set up pre-folded (once the arm64 contpte support is
>> added in the next few patches). This leads to noticeable performance
>> improvement during split.
>>
> Acked-by: David Hildenbrand <david@...hat.com>

Thanks!