On Thu, 28 Jan 2021, David Hildenbrand wrote: > > On Thu, 28 Jan 2021, David Hildenbrand wrote: > > > >> diff --git a/mm/vmstat.c b/mm/vmstat.c > >> index 7758486097f9..957680db41fa 100644 > >> --- a/mm/vmstat.c > >> +++ b/mm/vmstat.c > >> @@ -1650,6 +1650,11 @@ static void zoneinfo_show_print(struct seq_file *m, pg_data_t *pgdat, > >> zone->spanned_pages, > >> zone->present_pages, > >> zone_managed_pages(zone)); > >> +#ifdef CONFIG_CMA > >> + seq_printf(m, > >> + "\n cma %lu", > >> + zone->cma_pages); > >> +#endif > >> > >> seq_printf(m, > >> "\n protection: (%ld", > > > > Hmm, not sure about this. If cma is only printed for CONFIG_CMA, we can't > > distinguish between (1) a kernel without your patch without including some > > version checking and (2) a kernel without CONFIG_CMA enabled. IOW, > > "cma 0" carries value: we know immediately that we do not have any CMA > > pages on this zone, period. > > > > /proc/zoneinfo is also not known for its conciseness so I think printing > > "cma 0" even for !CONFIG_CMA is helpful :) > > > > I think this #ifdef should be removed and it should call into a > > zone_cma_pages(struct zone *zone) which returns 0UL if disabled. > > > > Yeah, that’s also what I proposed in a sub-thread here. > Ah, I certainly think your original intuition was correct. > The last option would be going the full mile and not printing nr_free_cma. Code might get a bit uglier though, but we could also remove that stats counter ;) > > I don‘t particularly care, while printing „0“ might be easier, removing nr_free_cma might be cleaner. > > But then, maybe there are tools that expect that value to be around on any kernel? > Yeah, that's probably undue risk, the ship has sailed and there's no significant upside. I still think "cma 0" in /proc/zoneinfo carries value, though, especially for NUMA and it looks like this is how it's done in linux-next. With a single read of the file, userspace can make the determination what CMA pages exist on this node. In general, I think the rule-of-thumb is that the fewer ifdefs in /proc/zoneinfo, the easier it is for userspace to parse it. (I made that change to /proc/zoneinfo to even print non-existant zones for each node because otherwise you cannot determine what the indices of things like vm.lowmem_reserve_ratio represent.)
