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: <CAJD7tkY0zzwX1BCbayKSXSxwKEGiEJzzKggP8dJccdajsr_bKw@mail.gmail.com>
Date: Mon, 14 Oct 2024 16:48:41 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: akpm@...ux-foundation.org, kent.overstreet@...ux.dev, corbet@....net, 
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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 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?

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?