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Message-ID: <79234eac-d7cc-424b-984d-b78861a5e862@126.com>
Date: Tue, 30 Jul 2024 08:57:52 +0800
From: Ge Yang <yangge1116@....com>
To: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, stable@...r.kernel.org,
21cnbao@...il.com, baolin.wang@...ux.alibaba.com, liuzixing@...on.cn
Subject: Re: [PATCH V2] mm/gup: Clear the LRU flag of a page before adding to
LRU batch
在 2024/7/29 22:04, David Hildenbrand 写道:
> On 22.06.24 08:48, yangge1116@....com wrote:
>> From: yangge <yangge1116@....com>
>>
>> If a large number of CMA memory are configured in system (for example,
>> the
>> CMA memory accounts for 50% of the system memory), starting a virtual
>> virtual machine, it will call pin_user_pages_remote(..., FOLL_LONGTERM,
>> ...) to pin memory. Normally if a page is present and in CMA area,
>> pin_user_pages_remote() will migrate the page from CMA area to non-CMA
>> area because of FOLL_LONGTERM flag. But the current code will cause the
>> migration failure due to unexpected page refcounts, and eventually cause
>> the virtual machine fail to start.
>>
>> If a page is added in LRU batch, its refcount increases one, remove the
>> page from LRU batch 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 LRU batch in case the page is in it,
>> however, folio_test_lru() is not sufficient to tell whether the page is
>> in LRU batch or not, if the page is in LRU batch, the migration will
>> fail.
>>
>> To solve the problem above, we modify the logic of adding to LRU batch.
>> Before adding a page to LRU batch, we clear the LRU flag of the page so
>> that we can check whether the page is in LRU batch by
>> folio_test_lru(page).
>> Seems making the LRU flag of the page invisible a long time is no
>> problem,
>> because a new page is allocated from buddy and added to the lru batch,
>> its LRU flag is also not visible for a long time.
>>
>> Cc: <stable@...r.kernel.org>
>> Signed-off-by: yangge <yangge1116@....com>
>> ---
>> mm/swap.c | 43 +++++++++++++++++++++++++++++++------------
>> 1 file changed, 31 insertions(+), 12 deletions(-)
>>
>> diff --git a/mm/swap.c b/mm/swap.c
>> index dc205bd..9caf6b0 100644
>> --- a/mm/swap.c
>> +++ b/mm/swap.c
>> @@ -211,10 +211,6 @@ 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);
>> @@ -255,11 +251,16 @@ 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)) {
>> + !folio_test_unevictable(folio)) {
>> struct folio_batch *fbatch;
>> unsigned long flags;
>> folio_get(folio);
>> + if (!folio_test_clear_lru(folio)) {
>> + folio_put(folio);
>> + return;
>> + }
>> +
>> local_lock_irqsave(&lru_rotate.lock, flags);
>> fbatch = this_cpu_ptr(&lru_rotate.fbatch);
>> folio_batch_add_and_move(fbatch, folio, lru_move_tail_fn);
>> @@ -352,11 +353,15 @@ 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)) {
>> + if (!folio_test_active(folio) && !folio_test_unevictable(folio)) {
>> struct folio_batch *fbatch;
>> folio_get(folio);
>> + if (!folio_test_clear_lru(folio)) {
>> + folio_put(folio);
>> + return;
>> + }
>> +
>> local_lock(&cpu_fbatches.lock);
>> fbatch = this_cpu_ptr(&cpu_fbatches.activate);
>> folio_batch_add_and_move(fbatch, folio, folio_activate_fn);
>> @@ -700,6 +705,11 @@ void deactivate_file_folio(struct folio *folio)
>> return;
>> folio_get(folio);
>> + if (!folio_test_clear_lru(folio)) {
>> + folio_put(folio);
>> + return;
>> + }
>> +
>> local_lock(&cpu_fbatches.lock);
>> fbatch = this_cpu_ptr(&cpu_fbatches.lru_deactivate_file);
>> folio_batch_add_and_move(fbatch, folio, lru_deactivate_file_fn);
>> @@ -716,11 +726,16 @@ 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())) {
>> + if (!folio_test_unevictable(folio) && (folio_test_active(folio) ||
>> + lru_gen_enabled())) {
>> struct folio_batch *fbatch;
>> folio_get(folio);
>> + if (!folio_test_clear_lru(folio)) {
>> + folio_put(folio);
>> + return;
>> + }
>> +
>> local_lock(&cpu_fbatches.lock);
>> fbatch = this_cpu_ptr(&cpu_fbatches.lru_deactivate);
>> folio_batch_add_and_move(fbatch, folio, lru_deactivate_fn);
>> @@ -737,12 +752,16 @@ void folio_deactivate(struct folio *folio)
>> */
>> 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)) {
>> + if (folio_test_anon(folio) && folio_test_swapbacked(folio) &&
>> + !folio_test_swapcache(folio) &&
>> !folio_test_unevictable(folio)) {
>> struct folio_batch *fbatch;
>> folio_get(folio);
>> + if (!folio_test_clear_lru(folio)) {
>> + folio_put(folio);
>> + return;
>> + }
>
> Looking at this in more detail, I wonder if we can turn that to
>
> if (!folio_test_clear_lru(folio))
> return;
> folio_get(folio);
>
> In all cases? The caller must hold a reference, so this should be fine.
>
Seems the caller madvise_free_pte_range(...), calling
folio_mark_lazyfree(...), doesn't hold a reference on folio.
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