Re: [PATCH] mm: page-writeback: simplify memcg handling in test_clear_page_writeback()

[Hugh Dickins]

On Tue, 9 Feb 2021, Johannes Weiner wrote:

> Page writeback doesn't hold a page reference, which allows truncate to
> free a page the second PageWriteback is cleared. This used to require
> special attention in test_clear_page_writeback(), where we had to be
> careful not to rely on the unstable page->memcg binding and look up
> all the necessary information before clearing the writeback flag.
> 
> Since commit 073861ed77b6 ("mm: fix VM_BUG_ON(PageTail) and
> BUG_ON(PageWriteback)") test_clear_page_writeback() is called with an
> explicit reference on the page, and this dance is no longer needed.
> 
> Use unlock_page_memcg() and dec_lruvec_page_stat() directly.

s/stat()/state()/

This is a nice cleanup: I hadn't seen that connection at all.

But I think you should take it further:
__unlock_page_memcg() can then be static in mm/memcontrol.c,
and its declarations deleted from include/linux/memcontrol.h?

And further: delete __dec_lruvec_state() and dec_lruvec_state()
from include/linux/vmstat.h - unless you feel that every "inc"
ought to be matched by a "dec", even when unused.

> 
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>

> ---
>  mm/page-writeback.c | 9 +++------
>  1 file changed, 3 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index eb34d204d4ee..f6c2c3165d4d 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2722,12 +2722,9 @@ EXPORT_SYMBOL(clear_page_dirty_for_io);
>  int test_clear_page_writeback(struct page *page)
>  {
>  	struct address_space *mapping = page_mapping(page);
> -	struct mem_cgroup *memcg;
> -	struct lruvec *lruvec;
>  	int ret;
>  
> -	memcg = lock_page_memcg(page);
> -	lruvec = mem_cgroup_page_lruvec(page, page_pgdat(page));
> +	lock_page_memcg(page);
>  	if (mapping && mapping_use_writeback_tags(mapping)) {
>  		struct inode *inode = mapping->host;
>  		struct backing_dev_info *bdi = inode_to_bdi(inode);
> @@ -2755,11 +2752,11 @@ int test_clear_page_writeback(struct page *page)
>  		ret = TestClearPageWriteback(page);
>  	}
>  	if (ret) {
> -		dec_lruvec_state(lruvec, NR_WRITEBACK);
> +		dec_lruvec_page_state(page, NR_WRITEBACK);
>  		dec_zone_page_state(page, NR_ZONE_WRITE_PENDING);
>  		inc_node_page_state(page, NR_WRITTEN);
>  	}
> -	__unlock_page_memcg(memcg);
> +	unlock_page_memcg(page);
>  	return ret;
>  }
>  
> -- 
> 2.30.0