Re: [PATCH v18 25/32] mm/mlock: remove lru_lock on TestClearPageMlocked in munlock_vma_page

[Hugh Dickins <hughd@google.com>]

On Mon, 24 Aug 2020, Alex Shi wrote:

> In the func munlock_vma_page, the page must be PageLocked as well as
> pages in split_huge_page series funcs. Thus the PageLocked is enough
> to serialize both funcs.
> 
> So we could relief the TestClearPageMlocked/hpage_nr_pages which are not
> necessary under lru lock.
> 
> As to another munlock func __munlock_pagevec, which no PageLocked
> protection and should remain lru protecting.
> 
> Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>

I made some comments on the mlock+munlock situation last week:
I won't review this 24/32 and 25/32 now, but will take a look
at your github tree tomorrow instead.  Perhaps I'll find you have
already done the fixes, perhaps I'll find you have merged these back
into earlier patches.  And I won't be reviewing beyond this point:
this is enough for now, I think.

Hugh

> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> ---
>  mm/mlock.c | 41 +++++++++++++++--------------------------
>  1 file changed, 15 insertions(+), 26 deletions(-)
> 
> diff --git a/mm/mlock.c b/mm/mlock.c
> index 0448409184e3..46a05e6ec5ba 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -69,9 +69,9 @@ void clear_page_mlock(struct page *page)
>  	 *
>  	 * See __pagevec_lru_add_fn for more explanation.
>  	 */
> -	if (!isolate_lru_page(page)) {
> +	if (!isolate_lru_page(page))
>  		putback_lru_page(page);
> -	} else {
> +	else {
>  		/*
>  		 * We lost the race. the page already moved to evictable list.
>  		 */
> @@ -178,7 +178,6 @@ static void __munlock_isolation_failed(struct page *page)
>  unsigned int munlock_vma_page(struct page *page)
>  {
>  	int nr_pages;
> -	struct lruvec *lruvec;
>  
>  	/* For try_to_munlock() and to serialize with page migration */
>  	BUG_ON(!PageLocked(page));
> @@ -186,37 +185,22 @@ unsigned int munlock_vma_page(struct page *page)
>  	VM_BUG_ON_PAGE(PageTail(page), page);
>  
>  	/*
> -	 * Serialize split tail pages in __split_huge_page_tail() 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().
> -	 * TestClearPageLRU can't be used here to block page isolation, since
> -	 * out of lock clear_page_mlock may interfer PageLRU/PageMlocked
> -	 * sequence, same as __pagevec_lru_add_fn, and lead the page place to
> -	 * wrong lru list here. So relay on PageLocked to stop lruvec change
> -	 * in mem_cgroup_move_account().
> +	 * Serialize split tail pages in __split_huge_page_tail() by
> +	 * lock_page(); Do TestClearPageMlocked/PageLRU sequence like
> +	 * clear_page_mlock().
>  	 */
> -	lruvec = lock_page_lruvec_irq(page);
> -
> -	if (!TestClearPageMlocked(page)) {
> +	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);
>  
> -	if (__munlock_isolate_lru_page(page, lruvec, true)) {
> -		unlock_page_lruvec_irq(lruvec);
> +	if (!isolate_lru_page(page))
>  		__munlock_isolated_page(page);
> -		goto out;
> -	}
> -	__munlock_isolation_failed(page);
> -
> -unlock_out:
> -	unlock_page_lruvec_irq(lruvec);
> +	else
> +		__munlock_isolation_failed(page);
>  
> -out:
>  	return nr_pages - 1;
>  }
>  
> @@ -305,6 +289,11 @@ static void __munlock_pagevec(struct pagevec *pvec, struct zone *zone)
>  
>  		/* block memcg change in mem_cgroup_move_account */
>  		lock_page_memcg(page);
> +		/*
> +		 * Serialize split tail pages in __split_huge_page_tail() which
> +		 * might otherwise copy PageMlocked to part of the tail pages
> +		 * before we clear it in the head page.
> +		 */
>  		lruvec = relock_page_lruvec_irq(page, lruvec);
>  		if (TestClearPageMlocked(page)) {
>  			/*
> -- 
> 1.8.3.1
> 
>