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Thu, 3 Jul 2025 09:41:38 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id zCwCE1JQZmjGBQAAD6G6ig (envelope-from ); Thu, 03 Jul 2025 09:41:38 +0000 Date: Thu, 3 Jul 2025 11:41:36 +0200 From: Oscar Salvador To: Dev Jain Cc: akpm@linux-foundation.org, david@redhat.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, baohua@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] khugepaged: Reduce race probability between migration and khugepaged Message-ID: References: <20250703054823.49149-1-dev.jain@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250703054823.49149-1-dev.jain@arm.com> X-Rspamd-Action: no action X-Stat-Signature: i8z8sdctmr6hm4ezuetf33gjwbnrra1k X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 8740F140003 X-Rspam-User: X-HE-Tag: 1751535700-241923 X-HE-Meta: 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 4OiOb8Mz xYvsuJ725NiARFfDLMigBBhaIrn5y6qes2grPO5262htpI/QDAN1Z7vrm6o3LRsrHJ/Dq4co3HTdXeaKjsfGFDjil35ULHg+ZNUmJSBZfwSryfClgEzH2lnhrsfpTxvqS8mf9mxzs9+G1RofI4PaEkAGC8VXiPQbLSk5DOzWGPQXs3aWqNykHAiSQ1nnyqElPN42igS9mj0per7bqjSlDLmsTRMXoDlBj9XaW2wnXxri5bT8U16Z9yUiNu/c0R5jU25T7zu4UZw4eMgg1/ZbS/o1fMSEwomr/aqeiKslU8YiNZ7taOA9WYt41ZEuMXtv5f9hFjPGUQq5txZM= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Jul 03, 2025 at 11:18:23AM +0530, Dev Jain wrote: > Suppose a folio is under migration, and khugepaged is also trying to > collapse it. collapse_pte_mapped_thp() will retrieve the folio from the > page cache via filemap_lock_folio(), thus taking a reference on the folio > and sleeping on the folio lock, since the lock is held by the migration > path. Migration will then fail in > __folio_migrate_mapping -> folio_ref_freeze. Reduce the probability of > such a race happening (leading to migration failure) by bailing out > if we detect a PMD is marked with a migration entry. > > This fixes the migration-shared-anon-thp testcase failure on Apple M3. > > Note that, this is not a "fix" since it only reduces the chance of > interference of khugepaged with migration, wherein both the kernel > functionalities are deemed "best-effort". > > Signed-off-by: Dev Jain David's comment refering to a 'present PMD entry' seems more clear to me, but Acked-by: Oscar Salvador -- Oscar Salvador SUSE Labs