linux-kernel - Re: [RFC PATCH v2 3/3] mm: mlock: update mlock_pte

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Date:   Mon, 17 Jul 2023 16:12:39 +0800
From:   Yin Fengwei <fengwei.yin@...el.com>
To:     Yu Zhao <yuzhao@...gle.com>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <akpm@...ux-foundation.org>, <willy@...radead.org>,
        <david@...hat.com>, <ryan.roberts@....com>, <shy828301@...il.com>
Subject: Re: [RFC PATCH v2 3/3] mm: mlock: update mlock_pte_range to handle
 large folio



On 7/17/23 08:35, Yu Zhao wrote:
> On Sun, Jul 16, 2023 at 6:00 PM Yin, Fengwei <fengwei.yin@...el.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:

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