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: <CAJuCfpGV3hwCRJj6D-SnSOc+VEe5=_045R1aGJEuYCL7WESsrg@mail.gmail.com>
Date: Fri, 18 Oct 2024 09:04:24 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Michal Hocko <mhocko@...e.com>
Cc: David Hildenbrand <david@...hat.com>, John Hubbard <jhubbard@...dia.com>, 
	Yosry Ahmed <yosryahmed@...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 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.

I'll change the first one to also specifically guide the user to
disable CONFIG_PGALLOC_TAG_USE_PAGEFLAGS.
When these happen, memory profiling gets disabled automatically. These
two cases would be the main ones when the user would want to disable
CONFIG_PGALLOC_TAG_USE_PAGEFLAGS to keep memory profiling enabled.
I also think when we auto-disable memory profiling at runtime like
that, I should make /proc/allocinfo empty so that it's apparent it is
disabled and the user does not use stale data.


> --
> Michal Hocko
> SUSE Labs