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: <ZxJcryjDUk_LzOuj@tiehlicka>
Date: Fri, 18 Oct 2024 15:03:43 +0200
From: Michal Hocko <mhocko@...e.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: David Hildenbrand <david@...hat.com>,
	John Hubbard <jhubbard@...dia.com>,
	Yosry Ahmed <yosryahmed@...gle.com>, akpm@...ux-foundation.org,
	kent.overstreet@...ux.dev, corbet@....net, arnd@...db.de,
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	thuth@...hat.com, tglx@...utronix.de, bp@...en8.de,
	xiongwei.song@...driver.com, ardb@...nel.org, vbabka@...e.cz,
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Subject: Re: [PATCH v3 5/5] alloc_tag: config to store page allocation tag
 refs in page flags

On Tue 15-10-24 08:58:59, Suren Baghdasaryan wrote:
> On Tue, Oct 15, 2024 at 8:42 AM David Hildenbrand <david@...hat.com> wrote:
[...]
> > Right, I think what John is concerned about (and me as well) is that
> > once a new feature really needs a page flag, there will be objection
> > like "no you can't, we need them for allocation tags otherwise that
> > feature will be degraded".
> 
> I do understand your concern but IMHO the possibility of degrading a
> feature should not be a reason to always operate at degraded capacity
> (which is what we have today). If one is really concerned about
> possible future regression they can set
> CONFIG_PGALLOC_TAG_USE_PAGEFLAGS=n and keep what we have today. That's
> why I'm strongly advocating that we do need
> CONFIG_PGALLOC_TAG_USE_PAGEFLAGS so that the user has control over how
> this scarce resource is used.

I really do not think users will know how/why to setup this and I wouldn't
even bother them thinking about that at all TBH. 

This is an implementation detail. It is fine to reuse unused flags space
as a storage as a performance optimization but why do you want users to
bother with that? Why would they ever want to say N here?
-- 
Michal Hocko
SUSE Labs