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: <Zw3H_7mlrqxRwz_H@casper.infradead.org>
Date: Tue, 15 Oct 2024 02:40:15 +0100
From: Matthew Wilcox <willy@...radead.org>
To: John Hubbard <jhubbard@...dia.com>
Cc: Yosry Ahmed <yosryahmed@...gle.com>,
	Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
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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 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.