linux-kernel - Re: [PATCH] selftests/mm: add test for (BATCH_PROCESS)MADV

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Message-ID: <e80fa23e-f659-4eef-89ba-8c9f5578b78e@redhat.com>
Date: Mon, 23 Jun 2025 15:49:05 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 wang lian <lianux.mm@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Shuah Khan <shuah@...nel.org>,
 Christian Brauner <brauner@...nel.org>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
 SeongJae Park <sj@...nel.org>, zijing.zhang@...ton.me, ryncsn@...il.com,
 p1ucky0923@...il.com, gkwang@...x-info.com,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka
 <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH] selftests/mm: add test for (BATCH_PROCESS)MADV_DONTNEED

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