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

[John Hubbard]

On 10/14/24 4:48 PM, Yosry Ahmed wrote:
> On Mon, Oct 14, 2024 at 1:37 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> 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?