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

[Yosry Ahmed]

On Mon, Oct 14, 2024 at 4:53 PM John Hubbard <jhubbard@xxxxxxxxxx> wrote:
>
> 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.

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.