Re: [PATCH V2] mm/gup: Clear the LRU flag of a page before adding to LRU batch

[David Hildenbrand <david@redhat.com>]

On 22.06.24 08:48, yangge1116@126.com wrote:
> From: yangge <yangge1116@126.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@vger.kernel.org>
> Signed-off-by: yangge <yangge1116@126.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.

-- 
Cheers,

David / dhildenb