linux-kernel - Re: [PATCH v3 5/5] alloc_tag: config to store page allocation tag refs in page flags

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Message-ID: <ba888da6-cd45-41b6-9d97-8292474d3ce6@nvidia.com>
Date: Mon, 14 Oct 2024 17:03:32 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
CC: Suren Baghdasaryan <surenb@...gle.com>, <akpm@...ux-foundation.org>,
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Subject: Re: [PATCH v3 5/5] alloc_tag: config to store page allocation tag
 refs in page flags

On 10/14/24 4:56 PM, Yosry Ahmed wrote:
> On Mon, Oct 14, 2024 at 4:53 PM John Hubbard <jhubbard@...dia.com> wrote:
>>
>> On 10/14/24 4:48 PM, Yosry Ahmed wrote:
>>> On Mon, Oct 14, 2024 at 1:37 PM Suren Baghdasaryan <surenb@...gle.com> wrote:
>>>>
>>>> Add CONFIG_PGALLOC_TAG_USE_PAGEFLAGS to store allocation tag
>>>> references directly in the page flags. This eliminates memory
>>>> overhead caused by page_ext and results in better performance
>>>> for page allocations.
>>>> If the number of available page flag bits is insufficient to
>>>> address all kernel allocations, profiling falls back to using
>>>> page extensions with an appropriate warning to disable this
>>>> config.
>>>> If dynamically loaded modules add enough tags that they can't
>>>> be addressed anymore with available page flag bits, memory
>>>> profiling gets disabled and a warning is issued.
>>>
>>> Just curious, why do we need a config option? If there are enough bits
>>> in page flags, why not use them automatically or fallback to page_ext
>>> otherwise?
>>
>> Or better yet, *always* fall back to page_ext, thus leaving the
>> scarce and valuable page flags available for other features?
>>
>> Sorry Suren, to keep coming back to this suggestion, I know
>> I'm driving you crazy here! But I just keep thinking it through
>> and failing to see why this feature deserves to consume so
>> many page flags.
> 
> I think we already always use page_ext today. My understanding is that
> the purpose of this series is to give the option to avoid using
> page_ext if there are enough unused page flags anyway, which reduces
> memory waste and improves performance.
> 
> My question is just why not have that be the default behavior with a
> config option, use page flags if there are enough unused bits,
> otherwise use page_ext.

I agree that if you're going to implement this feature at all, then
keying off of CONFIG_MEM_ALLOC_PROFILING seems sufficient, and no
need to add CONFIG_PGALLOC_TAG_USE_PAGEFLAGS on top of that.

thanks,
-- 
John Hubbard