Re: [RFC PATCH v3 2/8] mm: compaction: handle non-lru compound pages properly in isolate_migratepages_block().

[David Hildenbrand]

On 05.01.22 22:47, Zi Yan wrote:
> From: Zi Yan <ziy@xxxxxxxxxx>
> 
> In isolate_migratepages_block(), a !PageLRU tail page can be encountered
> when the page is larger than a pageblock. Use compound head page for the
> checks inside and skip the entire compound page when isolation succeeds.
> 

This will currently never happen, due to the way we always isolate
MAX_ORDER -1 ranges, correct?

Better note that in the patch description, because currently it reads
like it's an actual fix "can be encountered".

> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> ---
>  mm/compaction.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index b4e94cda3019..ad9053fbbe06 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -979,19 +979,23 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  		 * Skip any other type of page
>  		 */
>  		if (!PageLRU(page)) {
> +			struct page *head = compound_head(page);
>  			/*
>  			 * __PageMovable can return false positive so we need
>  			 * to verify it under page_lock.
>  			 */
> -			if (unlikely(__PageMovable(page)) &&
> -					!PageIsolated(page)) {
> +			if (unlikely(__PageMovable(head)) &&
> +					!PageIsolated(head)) {
>  				if (locked) {
>  					unlock_page_lruvec_irqrestore(locked, flags);
>  					locked = NULL;
>  				}
>  
> -				if (!isolate_movable_page(page, isolate_mode))
> +				if (!isolate_movable_page(head, isolate_mode)) {
> +					low_pfn += (1 << compound_order(head)) - 1 - (page - head);
> +					page = head;
>  					goto isolate_success;
> +				}
>  			}
>  
>  			goto isolate_fail;


-- 
Thanks,

David / dhildenb