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: <cd848c5f-50cd-4834-a6dc-dff16c586e49@nvidia.com>
Date: Mon, 14 Oct 2024 16:53:10 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Yosry Ahmed <yosryahmed@...gle.com>,
 Suren Baghdasaryan <surenb@...gle.com>
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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 4:48 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.

thanks,
-- 
John Hubbard

> 
> Is the reason that dynamically loadable modules, where the user may
> know in advance that the page flags won't be enough with the modules
> loaded?