Re: [PATCH] mm/migrate: fix mlock state loss when migrating PTE-mapped large folios

[Wandun]

On 11/10/26 19:16, David Hildenbrand (Arm) wrote:
> On 11/10/26 07:27, Wandun wrote:
>>
>>
>> On 11/9/26 21:51, David Hildenbrand (Arm) wrote:
>>> On 11/9/26 10:08, Wandun Chen wrote:
>>>> From: Wandun Chen <chenwandun@xxxxxxxxxxx>
>>>>
>>>> try_to_migrate() clears PG_mlocked when unmapping the source folio.
>>>> For PTE-mapped large folios, remove_migration_pte() restores one PTE at
>>>> a time, but the rmap helpers only restore mlock for full-folio mappings.
>>>> This leaves the destination PTE-mapped large folio without PG_mlocked.
>>>>
>>>> Restore mlock once all PTEs of the destination folio are restored under
>>>> a single page table lock.
>>>>
>>>> Fixes: dc68badcede4 ("mm: mlock: update mlock_pte_range to handle large folio")
>>>> Signed-off-by: Wandun Chen <chenwandun@xxxxxxxxxxx>
>>>> ---
>>>>  mm/migrate.c | 10 +++++++++-
>>>>  1 file changed, 9 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>>> index 94033c222e04..140e291d4587 100644
>>>> --- a/mm/migrate.c
>>>> +++ b/mm/migrate.c
>>>> @@ -353,6 +353,7 @@ static bool remove_migration_pte(struct folio *folio,
>>>>  {
>>>>  	struct rmap_walk_arg *rmap_walk_arg = arg;
>>>>  	DEFINE_FOLIO_VMA_WALK(pvmw, rmap_walk_arg->folio, vma, addr, PVMW_SYNC | PVMW_MIGRATION);
>>>> +	unsigned long nr_ptes = 0;
>>>>  
>>>>  	while (page_vma_mapped_walk(&pvmw)) {
>>>>  		rmap_t rmap_flags = RMAP_NONE;
>>>> @@ -446,9 +447,16 @@ static bool remove_migration_pte(struct folio *folio,
>>>>  			else
>>>>  				folio_add_file_rmap_pte(folio, new, vma);
>>>>  			set_pte_at(vma->vm_mm, pvmw.address, pvmw.pte, pte);
>>>> +
>>>> +			nr_ptes++;
>>>>  		}
>>>> -		if (READ_ONCE(vma->vm_flags) & VM_LOCKED)
>>>> +		if (READ_ONCE(vma->vm_flags) & VM_LOCKED) {
>>>> +			if (folio_test_large(folio) &&
>>>> +			    nr_ptes == folio_nr_pages(folio) &&
>>>> +			    !(pvmw.flags & PVMW_PGTABLE_CROSSED))
>>>> +				mlock_vma_folio(folio, vma);
>>>>  			mlock_drain_local();
>>>> +		}
>>>
>>> I have the recollection that the next try_to_unmap() would fix that up? Or is
>>> that not the case?
>>
>> It is the case, try_to_unmap() can restore it. However, until reclaim scans
>> the folio, Mlocked in /proc/meminfo can remain undercounted, giving users
>> a misleading impression that less memory is locked.
>>
>> A similar fix was made in commit
>> 	ab521b4142aa ("mm/rmap: improve mlock tracking for large folios").
>>
>> and handles cases where an entire folio is added in one rmap operation.
>>
>> However, when a PTE-mapped large folio is migrated, remove_migration_pte()
>> restores PTEs individually, passing nr_pages = 1 to the rmap helpers,
>> as a result, PG_mlocked is not set on the destination folio.
>>
>> This patch handles that case once the full mapping has been restored,
>> rather than waiting for try_to_unmap() to restore the mlock state.
> 
> Please revise the patch description pointing all that out :)

Sure, I'll update the patch description in next version.

> 
> Note that ab521b4142aa wasn't a fix (no Fixes:) why should this one here be one?
> 
I also agree that the Fixes tag should be dropped.

I was hesitant about including Fixes tag in v1, as there was no single obvious
commit to reference. I eventually added it because I considered the Mlocked
undercount in /proc/meminfo an accounting bug.

Best regards,
Wandun