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Subject: Re: [PATCH 5/8] x86/clear_page: add clear_page_uncached()
To:     Borislav Petkov <bp@alien8.de>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        Andy Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Michal Hocko <mhocko@kernel.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, X86 ML <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Arnd Bergmann <arnd@arndb.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ira Weiny <ira.weiny@intel.com>,
        linux-arch <linux-arch@vger.kernel.org>
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 <20201015104047.GD11838@zn.tnic>
From:   Ankur Arora <ankur.a.arora@oracle.com>
Message-ID: <06032ba0-057e-3f56-c1bf-59373ae74e88@oracle.com>
Date:   Thu, 15 Oct 2020 14:40:06 -0700
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On 2020-10-15 3:40 a.m., Borislav Petkov wrote:
> 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.

I think that'll work well for GB pages, where the clear_pages() helper
has enough information to make a decision.

But, unless I'm missing something, I'm not sure how that would work for
say, a 1GB mm_populate() using 4K pages. The clear_page() (or clear_pages())
in that case would only see the 4K size.

But let me think about this more (and look at the callsites as you suggest.)

> 
> Looking at the callsites would give you a better idea I'd say.
Thanks, yeah that's a good idea. Let me go do that.

Ankur

> 
> Thx.
>