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

Oscar Salvador
2021-01-28T13:46:07Z

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.
Let us see what other thinks as well.

Btw, should linux-uapi be CCed, as /proc/vmstat layout will change?

--
Oscar Salvador
SUSE L3