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

[David Hildenbrand <david@redhat.com>]

On 30.06.25 09:55, Anshuman Khandual wrote:
> On 30/06/25 10:18 AM, 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.
> 
> Could the migration be re-attempted after such failure ? Seems like
> the migration failure here is traded for a scan failure instead.
> 
>>
>> This fixes the migration-shared-anon-thp testcase failure on Apple M3.
> 
> Could you please provide some more context why this test case was
> failing earlier and how does this change here fixes the problem ?
> 
>>
>> 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 <dev.jain@arm.com>
>> ---
>>
>> This patch was part of
>> https://lore.kernel.org/all/20250625055806.82645-1-dev.jain@arm.com/
>> but I have sent it separately on suggestion of Lorenzo, and also because
>> I plan to send the first two patches after David Hildenbrand's
>> folio_pte_batch series gets merged.
>>
>>   mm/khugepaged.c | 12 ++++++++++--
>>   1 file changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 1aa7ca67c756..99977bb9bf6a 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,
>> @@ -941,6 +942,8 @@ 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))
>> @@ -1502,9 +1505,12 @@ 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)
> Should mapped PMD and migrating PMD be treated equally while scanning ?

Wanted to ask the same thing I think: why not simply use 
SCAN_PMD_MAPPED? After all, the folio is already pmd-mapped, just not 
using a present entry but (temporarily) using a migration entry.

-- 
Cheers,

David / dhildenb