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

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

On 30/06/25 1:25 pm, 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.

We already re-attempt migration. See NR_MAX_MIGRATE_PAGES_RETRY and
NR_MAX_MIGRATE_ASYNC_RETRY. Also just before freezing the refcount,
we do a suitable refcount check in folio_migrate_mapping(). So the
race happens after this and folio_ref_freeze() in __folio_migrate_mapping(),
therefore the window for the race is already very small in the migration
path, but large in the khugepaged path.
  

>
>> 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 ?

IMHO the explanation I have given in the patch description is clear
and succinct: the testcase is failing due to the race. This patch
shortens the race window, and the test on this particular hardware
does not hit the race window again.

>
>> 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 ?

SCAN_PMD_MAPPED is used as an indicator to change result to SCAN_SUCCEED
in khugepaged_scan_mm_slot: after the call to collapse_pte_mapped_thp. And,
it is also used in madvise_collapse() to do ++thps which is used to set the
return value of madvise_collapse. So I think this approach will be wrong.

>
>>   		return result;
>>   
>>   	/*
>> @@ -2716,6 +2722,7 @@ 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
>> @@ -2802,6 +2809,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
>>   			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: