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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Lorenzo Stoakes , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, David Hildenbrand , Shuah Khan 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> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 63DA040003 X-Stat-Signature: jatx8ibtayuffa6k184af7r6jg9cng95 X-HE-Tag: 1789521321-362548 X-HE-Meta: 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 sYxye4m1 uAcX9YvYaygxweY7zD7LIPB48vmxKwVrsJMMf0X+xZQZBp1sa7L/ZTIUGqqGIpJafBXg9/cse4kiIzXSOJcHJTjqa2p+IFgvmW+nYVVuilv6C6c34kuAiL2WNGfXub5iSbBfzYkXrH1JGrPoY7QRgkWYFdhpocGPwW4KJYKKTg4Qjw75HRCokCmQVoXQ4FJmiRKwDiZhUKx/dZs+0q0QirS6zf15Ek3Ek3ViVCdArJs931uFmgryH2VijhHTEPrDSTSU8isPxzBlDfnttI6XRqOBxRs49h7Nz6s8VRHHE6AjgnnTNjfeSe+/3KQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, 15 Sep 2026 10:21:35 +0100 Yeoreum Yun 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?