Re: [PATCH] selftests/mm: add test for (BATCH_PROCESS)MADV_DONTNEED

[David Hildenbrand]

On 23.06.25 14:35, Lorenzo Stoakes wrote:
> +cc Liam, David, Vlastimil, Jann
>
> (it might not be obvious from get_maintainers.pl but please cc
> maintainers/reviewers of the thing you are adding a test for, thanks!)
>
> Overall I'm not in favour of us taking this patch.
>
> There are a number of issues with it (explained inline below), but those aside,
> it seems to be:
>
> - Checking whether a simple anon buffer of arbitrary size is zapped by
>    MADV_DONTNEED.
>
> - Printing out a dubious microbenchmark that seems to be mostly asserting that
>    fewer sycalls are faster when using process_madvise() locally.
>
> And I'm struggling to see the value of that.

We have other tests that should already severely break if MADV_DONTNEED 
doesn't work ... but sure, we could think about more elaborate 
functional tests when they provide a clear benefit. (zapping all kinds 
of memory types, anon/ksm/huge zeropage/pagecache/hugetlb/ ... and using 
/proc/self/pagemap to see if the page table mappings are already gone)

I don't think we have a lot of process_madvise selftests, right?

hugtlb handling that was added recently is already tested to some degree 
in hugetlb-madvise.c.

In general, I'm not a fan of selftests that measure syscall performance ...


--
Cheers,

David / dhildenb