linux-kernel - Re: [PATCH] ARM: mm: fix no-MMU ZERO

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Date:   Wed, 19 Oct 2022 18:37:59 +0200
From:   Giulio Benetti <giulio.benetti@...ettiengineering.com>
To:     Arnd Bergmann <arnd@...db.de>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     Russell King <linux@...linux.org.uk>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH] ARM: mm: fix no-MMU ZERO_PAGE() implementation

On 19/10/22 09:00, Arnd Bergmann wrote:
> On Wed, Oct 19, 2022, at 00:32, Giulio Benetti wrote:
>> On 18/10/22 20:35, Arnd Bergmann wrote:
>>> On Tue, Oct 18, 2022, at 19:44, Giulio Benetti wrote:
>>>> On 18/10/22 09:03, Arnd Bergmann wrote:
>>>>> In addition to your fix, I see that arm is the only architecture
>>>>> that defines 'empty_zero_page' as a pointer to the page, when
>>>>> everything else just makes it a pointer to the data itself,
>>>>> or an 'extern char empty_zero_page[]' array, which we may want
>>>>> to change for consistency.
>>>>
>>>> I was about doing it, but then I tought to move one piece at a time.
>>>
>>> Right, it would definitely be a separate patch, but it
>>> can be a series of two patches. We probably wouldn't need to
>>> backport the second patch that turns it into a static allocation.
>>
>> I've sent the patchset of 2:
>> https://lore.kernel.org/all/20221018222503.90118-1-giulio.benetti@benettiengineering.com/T/#t
>>
>> I'm wondering if it makes sense to send a patchset for all those
>> architectures that have only one zero page. I've seen that for example
>> loongarch has more than one. But for the others I find the array
>> approach more linear, with less code all around and a bit faster in term
>> of code execution(of course really few, but better than nothing) since
>> that array is in .bss, so it will be zeroed earlier during a long
>> "memset" where assembly instructions for zeroing 8 bytes at a time are
>> used. What about this?
> 
> The initial zeroing should not matter at all in terms of performance,
> I think the only question is whether one wants a single zero page
> to be used everywhere or one per NUMA node to give better locality
> for a cache miss.
> 
> My guess is that for a system with 4KB pages, all the data
> in the zero page are typically available in a CPU cache already,
> so it doesn't matter, but it's possible that some machines benefit
> from having per-node pages when the page size isn't tiny compared
> to the typical cache sizes.
> 
> We should probably not touch this for any of the other architectures.

Ok, thanks for the explanation!

Best regards
-- 
Giulio Benetti
CEO/CTO@...etti Engineering sas