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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: References: <20260915-fix_khugepagd_fail-v1-0-bb6f04c8759f@arm.com> <20260915181519.648c2962143b761cf02efd17@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260915181519.648c2962143b761cf02efd17@linux-foundation.org> X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 49D341C0006 X-Stat-Signature: wqoe54djagibenn4ay5gk3amz7csrdkb X-HE-Tag: 1789525303-990733 X-HE-Meta: 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 5wxhXQJ0 JqOgYmBwnEFngH08G3pqdEfvieKZhS6B46/fOLuzVzskfJQ11J64YLsZxFPsUItDRdnmFi5K+7uitWtJFa4BJUHpJysCk+JQ2GsH/UoX1+IKBmQBlqZUm9cT4TAHLxlq8j+RchqqsAVeVlqZoVVAY4uAhv51qrA1DT6Q2Pz6MrE49VzrBVy8NTwJA13X3AL8kq9Q3ZBH189+XFD56BICFA+hDaosZH+xD+Zox+OR+SWpmlhizKhKMqlMLjxYfmoBhcvF6OX48ulm6DipaGwA9BF97Jthr+DKfRkLx 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? If the test environment can be set up to control those timing vagaries, I believe in that such cases are meaningful for verifying the expected behavior. This patch sets up the conditions needed to make the result reliable, so I don't think this test case should be dismissed as a bad idea or deleted as inherently unreliable. -- Sincerely, Yeoreum Yun