linux-kernel - Re: [PATCH 5/8] x86/clear_page: add clear_page

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Date:   Thu, 15 Oct 2020 12:40:47 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Ankur Arora <ankur.a.arora@...cle.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Michal Hocko <mhocko@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ira Weiny <ira.weiny@...el.com>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH 5/8] x86/clear_page: add clear_page_uncached()

On Wed, Oct 14, 2020 at 08:21:57PM -0700, Ankur Arora wrote:
> Also, if we did extend clear_page() to take the page-size as parameter
> we still might not have enough information (ex. a 4K or a 2MB page that
> clear_page() sees could be part of a GUP of a much larger extent) to
> decide whether to go uncached or not.

clear_page* assumes 4K. All of the lowlevel asm variants do. So adding
the size there won't bring you a whole lot.

So you'd need to devise this whole thing differently. Perhaps have a
clear_pages() helper which decides based on size what to do: uncached
clearing or the clear_page() as is now in a loop.

Looking at the callsites would give you a better idea I'd say.

Thx.

-- 
Regards/Gruss,
    Boris.

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