Re: [PATCH v1 2/2] mm/page_alloc: count CMA pages per zone and print them in /proc/zoneinfo

[David Hildenbrand]

On 28.01.21 14:44, Oscar Salvador wrote:
> On Thu, Jan 28, 2021 at 11:43:41AM +0100, David Hildenbrand wrote:
> > > My knowledge of CMA tends to be quite low, actually I though that CMA
> > > was somehow tied to ZONE_MOVABLE.
> >
> > CMA is often placed into one of the kernel zones, but can also end up in the movable zone.
>
> Ok good to know.
>
> > > I see how tracking CMA pages per zona might give you a clue, but what do
> > > you mean by "might behave differently - even after some of these pages might
> > > already have been allocated"
> >
> > Assume you have 4GB in ZONE_NORMAL but 1GB is assigned for CMA. You actually only have 3GB available for random kernel allocations, not 4GB.
> >
> > Currently, you can only observe the free CMA pages, excluding any pages that are already allocated. Having that information how many CMA pages we have can be helpful - similar to what we already have in /proc/meminfo.
>
> I see, I agree that it can provide some guidance.
>
> > > I see that NR_FREE_CMA_PAGES is there even without CONFIG_CMA, as you
> > > said, but I am not sure about adding size to a zone unconditionally.
> > > I mean, it is not terrible as IIRC, the maximum MAX_NUMNODES can get
> > > is 1024, and on x86_64 that would be (1024 * 4 zones) * 8 = 32K.
> > > So not a big deal, but still.
> >
> > I'm asking myself how many such systems will run without
> > CONFIG_CMA in the future.
>
> I am not sure, my comment was just to point out that even the added size might
> not be that large, hiding it under CONFIG_CMA seemed the right thing to
> do.
>
> > > diff --git a/mm/vmstat.c b/mm/vmstat.c
> > > index 8ba0870ecddd..5757df4bfd45 100644
> > > --- a/mm/vmstat.c
> > > +++ b/mm/vmstat.c
> > > @@ -1559,13 +1559,15 @@ static void zoneinfo_show_print(struct seq_file *m, pg_data_t *pgdat,
> > >                      "\n        spanned  %lu"
> > >                      "\n        present  %lu"
> > >                      "\n        managed  %lu",
> > > +                  "\n        cma      %lu",
> > >                      zone_page_state(zone, NR_FREE_PAGES),
> > >                      min_wmark_pages(zone),
> > >                      low_wmark_pages(zone),
> > >                      high_wmark_pages(zone),
> > >                      zone->spanned_pages,
> > >                      zone->present_pages,
> > > -                  zone->managed_pages);
> > > +                  zone->managed_pages,
> > > +                  IS_ENABLED(CONFIG_CMA) ? zone->cma_pages : 0);
> > >           seq_printf(m,
> > >                      "\n        protection: (%ld",
> > >
> > >
> > > I do not see it that ugly, but just my taste.
> >
> > IIRC, that does not work. The compiler will still complain
> > about a missing struct members. We would have to provide a
> > zone_cma_pages() helper with some ifdefery.
>
> Of course, it seems I switched off my brain.
>
> > We could do something like this on top
> >
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -530,7 +530,9 @@ struct zone {
> >          atomic_long_t           managed_pages;
> >          unsigned long           spanned_pages;
> >          unsigned long           present_pages;
> > +#ifdef CONFIG_CMA
> >          unsigned long           cma_pages;
> > +#endif
> >          const char              *name;
> > diff --git a/mm/vmstat.c b/mm/vmstat.c
> > index 97fc32a53320..b753a64f099f 100644
> > --- a/mm/vmstat.c
> > +++ b/mm/vmstat.c
> > @@ -1643,7 +1643,10 @@ static void zoneinfo_show_print(struct seq_file *m, pg_data_t *pgdat,
> >                     "\n        spanned  %lu"
> >                     "\n        present  %lu"
> >                     "\n        managed  %lu"
> > -                  "\n        cma      %lu",
> > +#ifdef CONFIG_CMA
> > +                  "\n        cma      %lu"
> > +#endif
> > +                  "%s",
> >                     zone_page_state(zone, NR_FREE_PAGES),
> >                     min_wmark_pages(zone),
> >                     low_wmark_pages(zone),
> > @@ -1651,7 +1654,10 @@ static void zoneinfo_show_print(struct seq_file *m, pg_data_t *pgdat,
> >                     zone->spanned_pages,
> >                     zone->present_pages,
> >                     zone_managed_pages(zone),
> > -                  zone->cma_pages);
> > +#ifdef CONFIG_CMA
> > +                  zone->cma_pages,
> > +#endif
> > +                  "");
> >          seq_printf(m,
> >                     "\n        protection: (%ld",
>
> Looks good to me, but I can see how those #ifdef can raise some
> eyebrows.

We could print it further above to avoid the "%s" ... "", or print it 
separately below. Then we'd only need a single ifdef. Might make sense

> Let us see what other thinks as well.
>
> Btw, should linux-uapi be CCed, as /proc/vmstat layout will change?

Is there a linux-uapi@ list? I know linux-api@ ("forum to discuss 
changes that affect the Linux programming interface (API or ABI)".

Good question, I can certainly cc linux-api@, although I doubt it's 
strictly necessary when adding something here.

Thanks!

--
Thanks,

David / dhildenb