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Date: Tue, 15 Sep 2026 18:15:19 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Yeoreum Yun <yeoreum.yun@arm.com>
Cc: Zi Yan <ziy@nvidia.com>, Baolin Wang <baolin.wang@linux.alibaba.com>,
 "Liam R. Howlett" <liam@infradead.org>, Nico Pache <nico.pache@linux.dev>,
 Ryan Roberts <ryan.roberts@arm.com>, Dev Jain <dev.jain@arm.com>, Barry
 Song <baohua@kernel.org>, Lance Yang <lance.yang@linux.dev>, Usama Arif
 <usama.arif@linux.dev>, Kiryl Shutsemau <kas@kernel.org>, Lorenzo Stoakes
 <ljs@kernel.org>, Vlastimil Babka <vbabka@kernel.org>, Mike Rapoport
 <rppt@kernel.org>, Suren Baghdasaryan <surenb@google.com>, Michal Hocko
 <mhocko@suse.com>, linux-mm@kvack.org, linux-kselftest@vger.kernel.org,
 linux-kernel@vger.kernel.org, David Hildenbrand <david@kernel.org>, Shuah
 Khan <shuah@kernel.org>
Subject: Re: [PATCH 0/2] kselftest: mm: fix intermittent failure khugepaged
 test
Message-Id: <20260915181519.648c2962143b761cf02efd17@linux-foundation.org>
In-Reply-To: <20260915-fix_khugepagd_fail-v1-0-bb6f04c8759f@arm.com>
References: <20260915-fix_khugepagd_fail-v1-0-bb6f04c8759f@arm.com>
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On Tue, 15 Sep 2026 10:21:35 +0100 Yeoreum Yun <yeoreum.yun@arm.com> wrote:

> There are intermittent failures in collapse_max_ptes_swap() and
> collapse_max_ptes_shared() when using the khugepaged_context:
> 
>   # Run test: collapse_max_ptes_shared (khugepaged:anon)
>   # Allocate huge page... OK
>   # Share huge page over fork()... OK
>   # Trigger CoW on page 1023 of 2048... OK
>   # Maybe collapse with max_ptes_shared exceeded.... OK
>   # Trigger CoW on page 1024 of 2048... Fail
>   Bail out! Unexpected huge page
>   # Planned tests != run tests (26 != 23)
>   # Totals: pass:23 fail:0 xfail:0 xpass:0 skip:0 error:0
> 
>   # Run test: collapse_max_ptes_swap (khugepaged:anon)
>   # Swapout 257 of 2048 pages... OK
>   # Maybe collapse with max_ptes_swap exceeded.... OK
>   # Swapout 256 of 2048 pages... OK
>   Bail out! Unexpected huge page
>   # Planned tests != run tests (26 != 17)
>   # Totals: pass:17 fail:0 xfail:0 xpass:0 skip:0 error:0
> 
> This happens because khugepaged may collapse the pages before wait_for_scan()
> is called, causing a sanity check that expects uncollapsed pages to fail.

Is this something we should be testing for?  If I'm understanding
correctly, success/failure is dependent on timing vargaries inside the
kernel and that isn't something we can reliably test for.

IOW, should we say "bad idea" and just delete this test case?