linux-kernel - Re: [PATCH v3 0/4] mm/folio_zero

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Message-ID: <AC2C5344-E655-45BB-B90B-D63C4AC8F2F6@nvidia.com>
Date: Tue, 15 Apr 2025 15:10:10 -0400
From: Zi Yan <ziy@...dia.com>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
 torvalds@...ux-foundation.org, akpm@...ux-foundation.org, bp@...en8.de,
 dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
 luto@...nel.org, peterz@...radead.org, paulmck@...nel.org,
 rostedt@...dmis.org, tglx@...utronix.de, willy@...radead.org,
 jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
 boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v3 0/4] mm/folio_zero_user: add multi-page clearing

On 13 Apr 2025, at 23:46, Ankur Arora wrote:

> This series adds multi-page clearing for hugepages. It is a rework
> of [1] which took a detour through PREEMPT_LAZY [2].
>
> Why multi-page clearing?: multi-page clearing improves upon the
> current page-at-a-time approach by providing the processor with a
> hint as to the real region size. A processor could use this hint to,
> for instance, elide cacheline allocation when clearing a large
> region.
>
> This optimization in particular is done by REP; STOS on AMD Zen
> where regions larger than L3-size use non-temporal stores.
>
> This results in significantly better performance.

Do you have init_on_alloc=1 in your kernel?
With that, pages coming from buddy allocator are zeroed
in post_alloc_hook() by kernel_init_pages(), which is a for loop
of clear_highpage_kasan_tagged(), a wrap of clear_page().
And folio_zero_user() is not used.

At least Debian, Fedora, and Ubuntu by default have
CONFIG_INIT_ON_ALLOC_DEFAULT_ON=y, which means init_on_alloc=1.

Maybe kernel_init_pages() should get your optimization as well,
unless you only target hugetlb pages.

Best Regards,
Yan, Zi