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

["David Hildenbrand (Arm)" <david@kernel.org>]

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@lixiang.com>
>>>
>>> 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@lixiang.com>
>>> ---
>>>  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 :)

Note that ab521b4142aa wasn't a fix (no Fixes:) why should this one here be one?

-- 
Cheers,

David