Re: [PATCH v2 5/6] alloc_tag: make page allocation tag reference size configurable

[Kent Overstreet]

On Tue, Sep 03, 2024 at 07:04:51PM GMT, Suren Baghdasaryan wrote:
> On Tue, Sep 3, 2024 at 6:17 PM Kent Overstreet
> <kent.overstreet@xxxxxxxxx> wrote:
> >
> > On Tue, Sep 03, 2024 at 06:07:28PM GMT, Suren Baghdasaryan wrote:
> > > On Sun, Sep 1, 2024 at 10:09 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > > >
> > > > On Sun,  1 Sep 2024 21:41:27 -0700 Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> > > >
> > > > > Introduce CONFIG_PGALLOC_TAG_REF_BITS to control the size of the
> > > > > page allocation tag references. When the size is configured to be
> > > > > less than a direct pointer, the tags are searched using an index
> > > > > stored as the tag reference.
> > > > >
> > > > > ...
> > > > >
> > > > > +config PGALLOC_TAG_REF_BITS
> > > > > +     int "Number of bits for page allocation tag reference (10-64)"
> > > > > +     range 10 64
> > > > > +     default "64"
> > > > > +     depends on MEM_ALLOC_PROFILING
> > > > > +     help
> > > > > +       Number of bits used to encode a page allocation tag reference.
> > > > > +
> > > > > +       Smaller number results in less memory overhead but limits the number of
> > > > > +       allocations which can be tagged (including allocations from modules).
> > > > > +
> > > >
> > > > In other words, "we have no idea what's best for you, you're on your
> > > > own".
> > > >
> > > > I pity our poor users.
> > > >
> > > > Can we at least tell them what they should look at to determine whether
> > > > whatever random number they chose was helpful or harmful?
> > >
> > > At the end of my reply in
> > > https://lore.kernel.org/all/CAJuCfpGNYgx0GW4suHRzmxVH28RGRnFBvFC6WO+F8BD4HDqxXA@xxxxxxxxxxxxxx/#t
> > > I suggested using all unused page flags. That would simplify things
> > > for the user at the expense of potentially using more memory than we
> > > need.
> >
> > Why would that use more memory, and how much?
> 
> Say our kernel uses 5000 page allocations and there are additional 100
> allocations from all the modules we are loading at runtime. They all
> can be addressed using 13 bits (8192 addressable tags), so the
> contiguous memory we will be preallocating to store these tags is 8192
> * sizeof(alloc_tag). sizeof(alloc_tag) is 40 bytes as of today but
> might increase in the future if we add more fields there for other
> uses (like gfp_flags for example). So, currently this would use 320KB.
> If we always use 16 bits we would be preallocating 2.5MB. So, that
> would be 2.2MB of wasted memory. Using more than 16 bits (65536
> addressable tags) will be impractical anytime soon (current number
> IIRC is a bit over 4000).

I see, it's not about the page bits, it's about the contiguous array of
alloc tags?

What if we just reserved address space, and only filled it in as needed?