Date: Wed, 5 Jun 2024 11:53:52 +0200
From: David Hildenbrand <david@...hat.com>
To: yangge1116 <yangge1116@....com>, akpm@...ux-foundation.org,
 Matthew Wilcox <willy@...radead.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 baolin.wang@...ux.alibaba.com, liuzixing@...on.cn
Subject: Re: [PATCH] mm/gup: don't check page lru flag before draining it

On 05.06.24 11:41, David Hildenbrand wrote:
> On 05.06.24 03:18, yangge1116 wrote:
>>
>>
>> 在 2024/6/4 下午9:47, David Hildenbrand 写道:
>>> On 04.06.24 12:48, yangge1116@....com wrote:
>>>> From: yangge <yangge1116@....com>
>>>>
>>>> If a page is added in pagevec, its ref count increases one, remove
>>>> the page from pagevec decreases one. Page migration requires the
>>>> page is not referenced by others except page mapping. Before
>>>> migrating a page, we should try to drain the page from pagevec in
>>>> case the page is in it, however, folio_test_lru() is not sufficient
>>>> to tell whether the page is in pagevec or not, if the page is in
>>>> pagevec, the migration will fail.
>>>>
>>>> Remove the condition and drain lru once to ensure the page is not
>>>> referenced by pagevec.
>>>
>>> What you are saying is that we might have a page on which
>>> folio_test_lru() succeeds, that was added to one of the cpu_fbatches,
>>> correct?
>>
>> Yes
>>
>>>
>>> Can you describe under which circumstances that happens?
>>>
>>
>> If we call folio_activate() to move a page from inactive LRU list to
>> active LRU list, the page is not only in LRU list, but also in one of
>> the cpu_fbatches.
>>
>> void folio_activate(struct folio *folio)
>> {
>>        if (folio_test_lru(folio) && !folio_test_active(folio) &&
>>            !folio_test_unevictable(folio)) {
>>            struct folio_batch *fbatch;
>>
>>            folio_get(folio);
>>            //After this, folio is in LRU list, and its ref count have
>> increased one.
>>
>>            local_lock(&cpu_fbatches.lock);
>>            fbatch = this_cpu_ptr(&cpu_fbatches.activate);
>>            folio_batch_add_and_move(fbatch, folio, folio_activate_fn);
>>            local_unlock(&cpu_fbatches.lock);
>>        }
>> }
> 
> Interesting, the !SMP variant does the folio_test_clear_lru().
> 
> It would be really helpful if we could reliably identify whether LRU
> batching code has a raised reference on a folio.
> 
> We have the same scenario in
> * folio_deactivate()
> * folio_mark_lazyfree()
> 
> In folio_batch_move_lru() we do the folio_test_clear_lru(folio).
> 
> No expert on that code, I'm wondering if we could move the
> folio_test_clear_lru() out, such that we can more reliably identify
> whether a folio is on the LRU batch or not.

I'm sure there would be something extremely broken with the following
(I don't know what I'm doing ;) ), but I wonder if there would be a way
to make something like that work (and perform well enough?).

diff --git a/mm/swap.c b/mm/swap.c
index 67786cb771305..642e471c3ec5a 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -212,10 +212,6 @@ static void folio_batch_move_lru(struct folio_batch *fbatch, move_fn_t move_fn)
         for (i = 0; i < folio_batch_count(fbatch); i++) {
                 struct folio *folio = fbatch->folios[i];
  
-               /* block memcg migration while the folio moves between lru */
-               if (move_fn != lru_add_fn && !folio_test_clear_lru(folio))
-                       continue;
-
                 folio_lruvec_relock_irqsave(folio, &lruvec, &flags);
                 move_fn(lruvec, folio);
  
@@ -255,8 +251,9 @@ static void lru_move_tail_fn(struct lruvec *lruvec, struct folio *folio)
   */
  void folio_rotate_reclaimable(struct folio *folio)
  {
-       if (!folio_test_locked(folio) && !folio_test_dirty(folio) &&
-           !folio_test_unevictable(folio) && folio_test_lru(folio)) {
+       if (folio_test_lru(folio) && !folio_test_locked(folio) &&
+           !folio_test_dirty(folio) && !folio_test_unevictable(folio) &&
+           folio_test_clear_lru(folio)) {
                 struct folio_batch *fbatch;
                 unsigned long flags;
  
@@ -354,7 +351,7 @@ static void folio_activate_drain(int cpu)
  void folio_activate(struct folio *folio)
  {
         if (folio_test_lru(folio) && !folio_test_active(folio) &&
-           !folio_test_unevictable(folio)) {
+           !folio_test_unevictable(folio) && folio_test_clear_lru(folio)) {
                 struct folio_batch *fbatch;
  
                 folio_get(folio);
@@ -699,6 +696,8 @@ void deactivate_file_folio(struct folio *folio)
         /* Deactivating an unevictable folio will not accelerate reclaim */
         if (folio_test_unevictable(folio))
                 return;
+       if (!folio_test_clear_lru(folio))
+               return;
  
         folio_get(folio);
         local_lock(&cpu_fbatches.lock);
@@ -718,7 +717,8 @@ void deactivate_file_folio(struct folio *folio)
  void folio_deactivate(struct folio *folio)
  {
         if (folio_test_lru(folio) && !folio_test_unevictable(folio) &&
-           (folio_test_active(folio) || lru_gen_enabled())) {
+           (folio_test_active(folio) || lru_gen_enabled()) &&
+           folio_test_clear_lru(folio)) {
                 struct folio_batch *fbatch;
  
                 folio_get(folio);
@@ -740,7 +740,8 @@ void folio_mark_lazyfree(struct folio *folio)
  {
         if (folio_test_lru(folio) && folio_test_anon(folio) &&
             folio_test_swapbacked(folio) && !folio_test_swapcache(folio) &&
-           !folio_test_unevictable(folio)) {
+           !folio_test_unevictable(folio) &&
+           folio_test_clear_lru(folio)) {
                 struct folio_batch *fbatch;
  
                 folio_get(folio);



-- 
Cheers,

David / dhildenb

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