Re: [PATCH v2 3/3] khugepaged: Reduce race probability between migration and khugepaged

[Dev Jain <dev.jain@arm.com>]

On 25/06/25 6:58 pm, Lorenzo Stoakes wrote:
> On Wed, Jun 25, 2025 at 11:28:06AM +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.
> Hm is this related to the series at all? Seems somewhat unrelated?

Not related.

>
> Is there a Fixes, Closes, etc.? Do we need something in stable?

We don't need anything. This is an "expected race" in the sense that
both migration and khugepaged collapse are best effort algorithms.
I am just seeing a test failure on my system because my system hits
the race more often. So this patch reduces the window for the race.

For some previous context, you may look at:

https://lore.kernel.org/all/20240801081657.1386743-1-dev.jain@arm.com/
and
https://lore.kernel.org/all/20240830051609.4037834-1-dev.jain@arm.com/

>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>> ---
>>   mm/khugepaged.c | 12 ++++++++++--
>>   1 file changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 4c8d33abfbd8..bc8774f62e86 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -31,6 +31,7 @@ enum scan_result {
>>   	SCAN_FAIL,
>>   	SCAN_SUCCEED,
>>   	SCAN_PMD_NULL,
>> +	SCAN_PMD_MIGRATION,
>>   	SCAN_PMD_NONE,
>>   	SCAN_PMD_MAPPED,
>>   	SCAN_EXCEED_NONE_PTE,
>> @@ -956,6 +957,8 @@ static inline int check_pmd_state(pmd_t *pmd)
>>
>>   	if (pmd_none(pmde))
>>   		return SCAN_PMD_NONE;
>> +	if (is_pmd_migration_entry(pmde))
>> +		return SCAN_PMD_MIGRATION;
>>   	if (!pmd_present(pmde))
>>   		return SCAN_PMD_NULL;
>>   	if (pmd_trans_huge(pmde))
>> @@ -1518,9 +1521,12 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>>   	    !range_in_vma(vma, haddr, haddr + HPAGE_PMD_SIZE))
>>   		return SCAN_VMA_CHECK;
>>
>> -	/* Fast check before locking page if already PMD-mapped */
>> +	/*
>> +	 * Fast check before locking folio if already PMD-mapped, or if the
>> +	 * folio is under migration
>> +	 */
>>   	result = find_pmd_or_thp_or_none(mm, haddr, &pmd);
>> -	if (result == SCAN_PMD_MAPPED)
>> +	if (result == SCAN_PMD_MAPPED || result == SCAN_PMD_MIGRATION)
>>   		return result;
>>
>>   	/*
>> @@ -2745,6 +2751,7 @@ static int madvise_collapse_errno(enum scan_result r)
>>   	case SCAN_PAGE_LRU:
>>   	case SCAN_DEL_PAGE_LRU:
>>   	case SCAN_PAGE_FILLED:
>> +	case SCAN_PMD_MIGRATION:
>>   		return -EAGAIN;
>>   	/*
>>   	 * Other: Trying again likely not to succeed / error intrinsic to
>> @@ -2834,6 +2841,7 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev,
>>   			goto handle_result;
>>   		/* Whitelisted set of results where continuing OK */
>>   		case SCAN_PMD_NULL:
>> +		case SCAN_PMD_MIGRATION:
>>   		case SCAN_PTE_NON_PRESENT:
>>   		case SCAN_PTE_UFFD_WP:
>>   		case SCAN_PAGE_RO:
>> --
>> 2.30.2
>>