linux-kernel - Re: [PATCH 0/1] mm: Optimizing hugepage zeroing in arm64

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Date:   Thu, 21 Jan 2021 17:46:16 +0000
From:   Will Deacon <will@...nel.org>
To:     Prathu Baronia <prathubaronia2011@...il.com>
Cc:     linux-kernel@...r.kernel.org, chintan.pandya@...plus.com,
        Prathu Baronia <prathu.baronia@...plus.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        "glider@...gle.com" <glider@...gle.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 0/1] mm: Optimizing hugepage zeroing in arm64

On Thu, Jan 21, 2021 at 10:21:50PM +0530, Prathu Baronia wrote:
> This patch removes the unnecessary kmap calls in the hugepage zeroing path and
> improves the timing by 62%.
> 
> I had proposed a similar change in Apr-May'20 timeframe in memory.c where I
> proposed to clear out a hugepage by directly calling a memset over the whole
> hugepage but got the opposition that the change was not architecturally neutral.
> 
> Upon revisiting this now I see significant improvement by removing around 2k
> barrier calls from the zeroing path. So hereby I propose an arm64 specific
> definition of clear_user_highpage().

Given that barrier() is purely a thing for the compiler, wouldn't the same
change yield a benefit on any other architecture without HIGHMEM? In which
case, I think this sort of change belongs in the core code if it's actually
worthwhile.

Will