Re: [PATCH v21 12/19] mm/mlock: remove lru_lock on TestClearPageMlocked

[Vlastimil Babka]

On 11/5/20 9:55 AM, Alex Shi wrote:
> In the func munlock_vma_page, comments mentained lru_lock needed for
> serialization with split_huge_pages. But the page must be PageLocked
> as well as pages in split_huge_page series funcs. Thus the PageLocked
> is enough to serialize both funcs.
>
> Further more, Hugh Dickins pointed: before splitting in
> split_huge_page_to_list, the page was unmap_page() to remove pmd/ptes
> which protect the page from munlock. Thus, no needs to guard
> __split_huge_page_tail for mlock clean, just keep the lru_lock there for
> isolation purpose.
>
> LKP found a preempt issue on __mod_zone_page_state which need change
> to mod_zone_page_state. Thanks!
>
> Signed-off-by: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Nit below:

> ---
>   mm/mlock.c | 26 +++++---------------------
>   1 file changed, 5 insertions(+), 21 deletions(-)
>
> diff --git a/mm/mlock.c b/mm/mlock.c
> index 884b1216da6a..796c726a0407 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -187,40 +187,24 @@ static void __munlock_isolation_failed(struct page *page)
>   unsigned int munlock_vma_page(struct page *page)
>   {
>   	int nr_pages;
> -	pg_data_t *pgdat = page_pgdat(page);
>   
>   	/* For try_to_munlock() and to serialize with page migration */

Now the reasons for locking are expanded?

>   	BUG_ON(!PageLocked(page));
> -
>   	VM_BUG_ON_PAGE(PageTail(page), page);
>   
> -	/*
> -	 * Serialize with any parallel __split_huge_page_refcount() which
> -	 * might otherwise copy PageMlocked to part of the tail pages before
> -	 * we clear it in the head page. It also stabilizes thp_nr_pages().
> -	 */
> -	spin_lock_irq(&pgdat->lru_lock);
> -
>   	if (!TestClearPageMlocked(page)) {
>   		/* Potentially, PTE-mapped THP: do not skip the rest PTEs */
> -		nr_pages = 1;
> -		goto unlock_out;
> +		return 0;
>   	}
>   
>   	nr_pages = thp_nr_pages(page);
> -	__mod_zone_page_state(page_zone(page), NR_MLOCK, -nr_pages);
> +	mod_zone_page_state(page_zone(page), NR_MLOCK, -nr_pages);
>   
> -	if (__munlock_isolate_lru_page(page, true)) {
> -		spin_unlock_irq(&pgdat->lru_lock);
> +	if (!isolate_lru_page(page))
>   		__munlock_isolated_page(page);
> -		goto out;
> -	}
> -	__munlock_isolation_failed(page);
> -
> -unlock_out:
> -	spin_unlock_irq(&pgdat->lru_lock);
> +	else
> +		__munlock_isolation_failed(page);
>   
> -out:
>   	return nr_pages - 1;
>   }
>