Re: [RFC PATCH v2 3/3] mm: mlock: update mlock_pte_range to handle large folio

[Yin Fengwei <fengwei.yin@intel.com>]

On 7/17/23 16:12, Yin Fengwei wrote:
> 
> 
> On 7/17/23 08:35, Yu Zhao wrote:
>> On Sun, Jul 16, 2023 at 6:00 PM Yin, Fengwei <fengwei.yin@intel.com> wrote:
>>>
>>> On 7/15/2023 2:06 PM, Yu Zhao wrote:
>>>> There is a problem here that I didn't have the time to elaborate: we
>>>> can't mlock() a folio that is within the range but not fully mapped
>>>> because this folio can be on the deferred split queue. When the split
>>>> happens, those unmapped folios (not mapped by this vma but are mapped
>>>> into other vmas) will be stranded on the unevictable lru.
>>>
>>> This should be fine unless I missed something. During large folio split,
>>> the unmap_folio() will be migrate(anon)/unmap(file) folio. Folio will be
>>> munlocked in unmap_folio(). So the head/tail pages will be evictable always.
>>
>> It's close but not entirely accurate: munlock can fail on isolated folios.
> 
> I suppose normal 4K page can hit this problem also and following patch could
> fix it:
No. This patch is not necessary as unevictable folio will not be picked up by
page reclaim. It's not possible to munlock the isolated folio from lru list.

The possible cases I am ware are: page_migrate, madvise and damon_pa_pageout and
lru_gen_look_around. The first three already handle this case correctly by call
  folio_putback_lru().

If folio is isolated, the split_folio() will just fail. So looks we are fine
for this corner case. Let me know if I miss something here.


Regards
Yin, Fengwei

> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 1080209a568bb..839b8398aa613 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2498,7 +2498,7 @@ static unsigned int move_folios_to_lru(struct lruvec *lruvec,
>  
>                 VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
>                 list_del(&folio->lru);
> -               if (unlikely(!folio_evictable(folio))) {
> +               if (unlikely(!folio_evictable(folio) || folio_test_unevictable(folio))) {
>                         spin_unlock_irq(&lruvec->lru_lock);
>                         folio_putback_lru(folio);
>                         spin_lock_irq(&lruvec->lru_lock);
> @@ -2723,7 +2723,7 @@ static void shrink_active_list(unsigned long nr_to_scan,
>                 folio = lru_to_folio(&l_hold);
>                 list_del(&folio->lru);
>  
> -               if (unlikely(!folio_evictable(folio))) {
> +               if (unlikely(!folio_evictable(folio) || folio_test_unevictable(folio))) {
>                         folio_putback_lru(folio);
>                         continue;
>                 }
> @@ -5182,7 +5182,7 @@ static int evict_folios(struct lruvec *lruvec, struct scan_control *sc, int swap
>         sc->nr_reclaimed += reclaimed;
>  
>         list_for_each_entry_safe_reverse(folio, next, &list, lru) {
> -               if (!folio_evictable(folio)) {
> +               if (!folio_evictable(folio) || folio_test_unevictable(folio)) {
>                         list_del(&folio->lru);
>                         folio_putback_lru(folio);
>                         continue;
> 
> 
> 
> Regards
> Yin, Fengwei
>