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

[Suren Baghdasaryan]

On Mon, Oct 14, 2024 at 5:03 PM 'John Hubbard' via kernel-team
<kernel-team@xxxxxxxxxxx> wrote:
>
> On 10/14/24 4:56 PM, Yosry Ahmed wrote:
> > On Mon, Oct 14, 2024 at 4:53 PM John Hubbard <jhubbard@xxxxxxxxxx> wrote:
> >>
> >> On 10/14/24 4:48 PM, Yosry Ahmed wrote:
> >>> On Mon, Oct 14, 2024 at 1:37 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> >>>>
> >>>> Add CONFIG_PGALLOC_TAG_USE_PAGEFLAGS to store allocation tag
> >>>> references directly in the page flags. This eliminates memory
> >>>> overhead caused by page_ext and results in better performance
> >>>> for page allocations.
> >>>> If the number of available page flag bits is insufficient to
> >>>> address all kernel allocations, profiling falls back to using
> >>>> page extensions with an appropriate warning to disable this
> >>>> config.
> >>>> If dynamically loaded modules add enough tags that they can't
> >>>> be addressed anymore with available page flag bits, memory
> >>>> profiling gets disabled and a warning is issued.
> >>>
> >>> Just curious, why do we need a config option? If there are enough bits
> >>> in page flags, why not use them automatically or fallback to page_ext
> >>> otherwise?
> >>
> >> 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.

Yosry's original guess was correct. If not for loadable modules we
could get away with having no CONFIG_PGALLOC_TAG_USE_PAGEFLAGS. We
could try to fit codetag references into page flags and if they do not
fit we could fall back to page_ext. That works fine when we have a
predetermined number of tags. But loadable modules make this number
variable at runtime and there is a possibility we run out of page flag
bits at runtime. In that case, the current patchset disables memory
profiling and issues a warning that the user should disable
CONFIG_PGALLOC_TAG_USE_PAGEFLAGS to avoid this situation. I expect
this to be a rare case but it can happen and we have to provide a way
for a user to avoid running out of bits, which is where
CONFIG_PGALLOC_TAG_USE_PAGEFLAGS would be used.
Thanks,
Suren.

>
> thanks,
> --
> John Hubbard
>
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