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Message-ID: <a59a2314-82f3-4278-a4de-338d725bbd32@arm.com>
Date: Tue, 19 Dec 2023 08:18:17 +0000
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Subject: Re: [PATCH v4 01/16] mm: thp: Batch-collapse PMD with set_ptes()
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To: David Hildenbrand <david@redhat.com>,
 Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>,
 Ard Biesheuvel <ardb@kernel.org>, Marc Zyngier <maz@kernel.org>,
 Oliver Upton <oliver.upton@linux.dev>, James Morse <james.morse@arm.com>,
 Suzuki K Poulose <suzuki.poulose@arm.com>, Zenghui Yu
 <yuzenghui@huawei.com>, Andrey Ryabinin <ryabinin.a.a@gmail.com>,
 Alexander Potapenko <glider@google.com>,
 Andrey Konovalov <andreyknvl@gmail.com>, Dmitry Vyukov <dvyukov@google.com>,
 Vincenzo Frascino <vincenzo.frascino@arm.com>,
 Andrew Morton <akpm@linux-foundation.org>,
 Anshuman Khandual <anshuman.khandual@arm.com>,
 Matthew Wilcox <willy@infradead.org>, Yu Zhao <yuzhao@google.com>,
 Mark Rutland <mark.rutland@arm.com>, Kefeng Wang
 <wangkefeng.wang@huawei.com>, John Hubbard <jhubbard@nvidia.com>,
 Zi Yan <ziy@nvidia.com>, Barry Song <21cnbao@gmail.com>,
 Alistair Popple <apopple@nvidia.com>, Yang Shi <shy828301@gmail.com>
Cc: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org
References: <20231218105100.172635-1-ryan.roberts@arm.com>
 <20231218105100.172635-2-ryan.roberts@arm.com>
 <8ce9f79c-be2f-4fa2-b356-39436a1d108a@redhat.com>
From: Ryan Roberts <ryan.roberts@arm.com>
In-Reply-To: <8ce9f79c-be2f-4fa2-b356-39436a1d108a@redhat.com>
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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@redhat.com>

Thanks!