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Wed, 23 Jul 2025 14:13:56 -0700 (PDT) X-Google-Smtp-Source: AGHT+IGHKu4d3onHAb7X8LWAPu0O+WUZ/aYhsMpnRlh9HgSTUlujJWY4TnCn/IV7CVGxpS2q71+P/Xaq3BIZIvRa+hU= X-Received: by 2002:a05:690c:600c:b0:710:f39f:a2bc with SMTP id 00721157ae682-719b4208ab4mr60589447b3.8.1753305236584; Wed, 23 Jul 2025 14:13:56 -0700 (PDT) MIME-Version: 1.0 References: <20250704040417.63826-1-dev.jain@arm.com> In-Reply-To: <20250704040417.63826-1-dev.jain@arm.com> From: Nico Pache Date: Wed, 23 Jul 2025 15:13:30 -0600 X-Gm-Features: Ac12FXwzjaonSZ_XBVSayzqL2leWBYkEuOCfl6SDlX5nA8rmqz_ZykeRteEcbLw Message-ID: Subject: Re: [PATCH v3] khugepaged: Reduce race probability between migration and khugepaged 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, ryan.roberts@arm.com, baohua@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Oscar Salvador , Anshuman Khandual X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: NnFJH3pFRcpWyg8uhwRB4tpvkuXCS_mlbwHcNZqC11w_1753305237 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: AFD0840004 X-Stat-Signature: z4b4e6f8hkpjbknqwm6j8fge5mgsg71e X-Rspam-User: X-Rspamd-Server: rspam07 X-HE-Tag: 1753305239-221462 X-HE-Meta: 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 ofN1ijHl H7+1cgZjdnvw1kVTm7x44RzE5C9wJm2I9eG7dPQMXxdTEIPCHZktuPIIdFpUX8kxsJbZK0ruuKo87JAPB5qQVB1jWnNbQxD1Ck3UjlirpS7JqAKKFlqTHe6/sFqrbbQNrwJY4EMpNm0AbZHx3yffs6wETfFw5aaXYOoKeP/iHFmTXpj0FGvx3eZXMWkr2LagLdoY43ZUOq0DVlzB9T0Z8Cf9fGiTm0gNdo0vpKrlASD2KvbLykH9ehQnSdMymDFSWWO2WTaNz9nbyQzbyUPxcwMb9WvIG17XpQnmCw8RmoQ98ljB5u4Bf9MlXlDpKOpAH+2iC9gQCepWJ4lLgwfnNPTgpZVQXOPxjPkh3JXpZpKs8J/5eFjaxUZbpCrhPMgozheaELQIt9n/LIVS1ipXUuSp5jw== 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 3, 2025 at 10:04=E2=80=AFPM 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". > > Acked-by: David Hildenbrand > Acked-by: Oscar Salvador > Reviewed-by: Anshuman Khandual > Reviewed-by: Zi Yan > Reviewed-by: Baolin Wang > Signed-off-by: Dev Jain LGTM! This is a nice check to have here :) Reviewed-by: Nico Pache > --- > > v2->v3: > - Improve comment (David) > > v1->v2: > - Remove SCAN_PMD_MIGRATION, merge into SCAN_PMD_MAPPED (David, Anshuman= ) > - Add a comment (Lorenzo) > > v1: > - https://lore.kernel.org/all/20250630044837.4675-1-dev.jain@arm.com/ > > mm/khugepaged.c | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 1aa7ca67c756..a55fb1dcd224 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -941,6 +941,14 @@ static inline int check_pmd_state(pmd_t *pmd) > > if (pmd_none(pmde)) > return SCAN_PMD_NONE; > + > + /* > + * The folio may be under migration when khugepaged is trying to > + * collapse it. Migration success or failure will eventually end > + * up with a present PMD mapping a folio again. > + */ > + if (is_pmd_migration_entry(pmde)) > + return SCAN_PMD_MAPPED; > if (!pmd_present(pmde)) > return SCAN_PMD_NULL; > if (pmd_trans_huge(pmde)) > -- > 2.30.2 >