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: <CAJD7tkY8LKVGN5QNy9q2UkRLnoOEd7Wcu_fKtxKqV7SN43QgrA@mail.gmail.com>
Date: Mon, 14 Oct 2024 16:56:32 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: John Hubbard <jhubbard@...dia.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org, kent.overstreet@...ux.dev, 
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Subject: Re: [PATCH v3 5/5] alloc_tag: config to store page allocation tag
 refs in page flags

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.