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: <ZxKWBfQ_Lps93fY1@tiehlicka>
Date: Fri, 18 Oct 2024 19:08:21 +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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Subject: Re: [PATCH v3 5/5] alloc_tag: config to store page allocation tag
 refs in page flags

On Fri 18-10-24 09:04:24, Suren Baghdasaryan wrote:
> On Fri, Oct 18, 2024 at 6:03 AM Michal Hocko <mhocko@...e.com> wrote:
> >
> > 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?
> 
> In this patch you can find a couple of warnings that look like this:
> 
> pr_warn("With module %s there are too many tags to fit in %d page flag
> bits. Memory profiling is disabled!\n", mod->name,
> NR_UNUSED_PAGEFLAG_BITS);
> emitted when we run out of page flag bits during a module loading,
> 
> pr_err("%s: alignment %lu is incompatible with allocation tag
> indexing, disable CONFIG_PGALLOC_TAG_USE_PAGEFLAGS",  mod->name,
> align);
> emitted when the arch-specific section alignment is incompatible with
> alloc_tag indexing.

You are asking users to workaround implementation issue by configuration
which sounds like a really bad idea. Why cannot you make the fallback
automatic?

-- 
Michal Hocko
SUSE Labs