Re: [PATCH v3 2/8] mm/mprotect: Remove NUMA_HUGE_PTE_UPDATES

[David Hildenbrand <david@redhat.com>]

On 15.07.24 21:21, Peter Xu wrote:
> In 2013, commit 72403b4a0fbd ("mm: numa: return the number of base pages
> altered by protection changes") introduced "numa_huge_pte_updates" vmstat
> entry, trying to capture how many huge ptes (in reality, PMD thps at that
> time) are marked by NUMA balancing.
> 
> This patch proposes to remove it for some reasons.
> 
> Firstly, the name is misleading. We can have more than one way to have a
> "huge pte" at least nowadays, and that's also the major goal of this patch,
> where it paves way for PUD handling in change protection code paths.
> 
> PUDs are coming not only for dax (which has already came and yet broken..),
> but also for pfnmaps and hugetlb pages.  The name will simply stop making
> sense when PUD will start to be involved in mprotect() world.
> 
> It'll also make it not reasonable either if we boost the counter for both
> pmd/puds.  In short, current accounting won't be right when PUD comes, so
> the scheme was only suitable at that point in time where PUD wasn't even
> possible.
> 
> Secondly, the accounting was simply not right from the start as long as it
> was also affected by other call sites besides NUMA.  mprotect() is one,
> while userfaultfd-wp also leverages change protection path to modify
> pgtables.  If it wants to do right it needs to check the caller but it
> never did; at least mprotect() should be there even in 2013.
> 
> It gives me the impression that nobody is seriously using this field, and
> it's also impossible to be serious.

It's weird and the implementation is ugly. The intention really was to 
only consider MM_CP_PROT_NUMA, but that apparently is not the case.

hugetlb/mprotect/... should have never been accounted.

[...]

>   
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 73d791d1caad..53656227f70d 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1313,7 +1313,6 @@ const char * const vmstat_text[] = {
>   
>   #ifdef CONFIG_NUMA_BALANCING
>   	"numa_pte_updates",
> -	"numa_huge_pte_updates",
>   	"numa_hint_faults",
>   	"numa_hint_faults_local",
>   	"numa_pages_migrated",

It's a user-visible update. I assume most tools should be prepared for 
this stat missing (just like handling !CONFIG_NUMA_BALANCING).

Apparently it's documented [1][2] for some distros:

"The amount of transparent huge pages that were marked for NUMA hinting 
faults. In combination with numa_pte_updates the total address space 
that was marked can be calculated."

And now I realize that change_prot_numa() would account these PMD 
updates as well in numa_pte_updates and I am confused about the SUSE 
documentation: "In combination with numa_pte_updates" doesn't really 
apply, right?

At this point I don't know what's right or wrong.

If we'd want to fix it instead, the right thing to do would be doing the 
accounting only with MM_CP_PROT_NUMA. But then, numa_pte_updates is also 
wrongly updated I believe :(


[1] 
https://documentation.suse.com/de-de/sles/12-SP5/html/SLES-all/cha-tuning-numactl.html
[2] 
https://support.oracle.com/knowledge/Oracle%20Linux%20and%20Virtualization/2749259_1.html

-- 
Cheers,

David / dhildenb