Re: [PATCH] mm/vma: predicate vma_assert_stabilised() on CONFIG_DEBUG_VM

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Tue, Nov 10, 2026 at 05:32:17PM +0000, Seunghyun An wrote:
> On Tue, Nov 10, 2026 at 03:10:40PM +0000, Lorenzo Stoakes (ARM) wrote:
> > I introduced commit 17fd82c3abe0 ("mm/vma: add and use
> > vma_assert_stabilised()") with the intent of picking up programmatic bugs
>
> Tested on an AMD EPYC 9634 (84 cores, 12 CCDs), CONFIG_DEBUG_VM=n.
> Applied on d48723711bae; base and patched were measured on the same day.
> Throughput figures are in faults/s; JVM pretouch time is in ms.
>
>                               base      patched
>   shared-VMA faults, 7 CCDs   1.776M    1.917M   +7.9%
>   shared-VMA faults, 12 CCDs  2.303M    2.431M   +5.6%
>   pft, 7 CCDs                 1.910M    2.126M  +11.3%
>   pft, 84 cores               4.201M    5.028M  +19.7%
>   JVM G1 pretouch (32G)       1877 ms   1672 ms -10.9%
>
> Tested-by: Seunghyun An <san55@wisc.edu>

Thanks!

>
> As a follow-up, I also checked whether avoiding the __vm_anon_pgoff_hi
> read in __folio_set_anon() provides any further benefit. For comparison,
> I tested my earlier patch 1 on top, which computes the index from
> vm_pgoff for anonymous VMAs and avoids that read:
>
>                               this patch  + patch 1
>   shared-VMA faults, 12 CCDs  2.431M      2.941M   +21%
>   pft, 12 CCDs                2.434M      3.474M   +43%
>
> With pft on 12 CCDs, this patch alone is essentially unchanged from
> base (2.442M faults/s). The additional improvement with patch 1 suggests
> that avoiding the __vm_anon_pgoff_hi read may be worth investigating on
> CPUs with high coherency costs between L3 cache domains.

OK that suggests there are cases other than the one you mentioned that have
an issue.

I'm disinclined to do something like that patch, as it's rather a hack.

Cna you give me the workload you're actually exercising here? Ideally
something that's easy for me to run + analyse locally.

I have a zen 5 with 8 CCDs so can probably exercise some stuff over here.

>
> Happy to run anything you'd like to try.

Thanks!

>
> Thanks,
> Seunghyun

--
Cheers, Lorenzo