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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 03 Jul 2025 10:27:38.0926 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: d5b44611-ae40-4767-b314-08ddba1c3bd1 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: AM1PEPF000252E1.eurprd07.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AS1PR08MB7634 On 03/07/25 3:39 pm, David Hildenbrand wrote: > On 03.07.25 11:52, Dev Jain wrote: >> >> On 03/07/25 2:55 pm, David Hildenbrand wrote: >>> On 03.07.25 07:48, 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 >>>> --- >>>> >>>> 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 | 9 +++++++++ >>>>    1 file changed, 9 insertions(+) >>>> >>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>>> index 1aa7ca67c756..3fdefc4f4984 100644 >>>> --- a/mm/khugepaged.c >>>> +++ b/mm/khugepaged.c >>>> @@ -941,6 +941,15 @@ 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 the PMD still pointing to a PMD-order folio, so return >>>> +     * SCAN_PMD_MAPPED. >>> >>> Nit: the last part (, so return ..) is obvious from the code. >>> >>> I would have written >>> >>> /* >>>   * 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 entry again. >>>   */ >>> >> Thanks for the suggestion, but >> >> PMD pointing to PMD-order folio necessarily implies present PMD entry, > > > but the converse is not true? For example it may point to a PTE > table. > > I see, talking about orders is confusing though. > > "with a present PMD mapping a folio again." Sure. Shall respin.