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Message-ID: <48150a28-ed48-49ff-9432-9cd30cda4da4@linux.alibaba.com>
Date: Wed, 5 Jun 2024 19:37:55 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: David Hildenbrand <david@...hat.com>, yangge1116 <yangge1116@....com>,
akpm@...ux-foundation.org, Matthew Wilcox <willy@...radead.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, liuzixing@...on.cn
Subject: Re: [PATCH] mm/gup: don't check page lru flag before draining it
On 2024/6/5 17:53, David Hildenbrand wrote:
> 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)) {
IMO, this seems violate the semantics of the LRU flag, since it's clear
that this folio is still in the LRU list.
With your changes, I think we should drain the page vectors before
calling folio_test_lru(), otherwise some cases will fail to check
folio_test_lru() if the folio remain in the page vectors for an extended
period.
> 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);
>
>
>
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