Re: [PATCH 02/13] mm: use page_off_lru()

[Yu Zhao <yuzhao@google.com>]

On Fri, Sep 18, 2020 at 09:37:00AM +0200, Michal Hocko wrote:
> On Thu 17-09-20 21:00:40, Yu Zhao wrote:
> > This patch replaces the only open-coded __ClearPageActive() with
> > page_off_lru(). There is no open-coded __ClearPageUnevictable()s.
> > 
> > Before this patch, we have:
> > 	__ClearPageActive()
> > 	add_page_to_lru_list()
> > 
> > After this patch, we have:
> > 	page_off_lru()
> > 		if PageUnevictable()
> > 			__ClearPageUnevictable()
> > 		else if PageActive()
> > 			__ClearPageActive()
> > 	add_page_to_lru_list()
> > 
> > Checking PageUnevictable() shouldn't be a problem because these two
> > flags are mutually exclusive. Leaking either will trigger bad_page().
> 
> I am sorry but the changelog is really hard to grasp. What are you
> trying to achieve, why and why it is safe. This should be a general
> outline for any patch. I have already commented on the previous patch
> and asked you for the explanation why removing __ClearPageActive from
> this path is desirable and safe. I have specifically asked to clarify
> the compound page situation as that is using its oen destructor in the
> freeing path and that might result in page_off_lru to be not called.

Haven't I explained we are NOT removing __ClearPageActive()? Is my
notion of the code structure above confusing you? Or 'open-coded'
could mean different things?

And I have asked this before: why does 'the compound page situation'
even matter here? Perhaps if you could give a concrete example related
to the code change and help me understand your concern?

> > Signed-off-by: Yu Zhao <yuzhao@google.com>
> > ---
> >  mm/vmscan.c | 6 +-----
> >  1 file changed, 1 insertion(+), 5 deletions(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 503fc5e1fe32..f257d2f61574 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1845,7 +1845,6 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
> >  	int nr_pages, nr_moved = 0;
> >  	LIST_HEAD(pages_to_free);
> >  	struct page *page;
> > -	enum lru_list lru;
> >  
> >  	while (!list_empty(list)) {
> >  		page = lru_to_page(list);
> > @@ -1860,14 +1859,11 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
> >  		lruvec = mem_cgroup_page_lruvec(page, pgdat);
> >  
> >  		SetPageLRU(page);
> > -		lru = page_lru(page);
> > -
> >  		add_page_to_lru_list(page, lruvec, lru);
> >  
> >  		if (put_page_testzero(page)) {
> >  			__ClearPageLRU(page);
> > -			__ClearPageActive(page);
> > -			del_page_from_lru_list(page, lruvec, lru);
> > +			del_page_from_lru_list(page, lruvec, page_off_lru(page));
> >  
> >  			if (unlikely(PageCompound(page))) {
> >  				spin_unlock_irq(&pgdat->lru_lock);
> > -- 
> > 2.28.0.681.g6f77f65b4e-goog
> 
> -- 
> Michal Hocko
> SUSE Labs