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

[Matthew Wilcox <willy@infradead.org>]

On Mon, Oct 14, 2024 at 05:03:32PM -0700, John Hubbard wrote:
> > > 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.
> 
> I agree that if you're going to implement this feature at all, then
> keying off of CONFIG_MEM_ALLOC_PROFILING seems sufficient, and no
> need to add CONFIG_PGALLOC_TAG_USE_PAGEFLAGS on top of that.

Maybe the right idea is to use all the bits which are unused in this
configuration for the first N callsites, then use page_ext for all the
ones larger than N.  It doesn't save any memory, but it does have the
performance improvement.