Re: [PATCH v7 02/10] mm/memcg: fold lru_lock in lock_page_lru

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jan 13, 2020 at 08:47:25PM +0800, Alex Shi wrote:
> 在 2020/1/13 下午5:55, Konstantin Khlebnikov 写道:
> >>> That's wrong. Here PageLRU must be checked again under lru_lock.
> >> Hi, Konstantin,
> >>
> >> For logical remain, we can get the lock and then release for !PageLRU.
> >> but I still can figure out the problem scenario. Would like to give more hints?
> > 
> > That's trivial race: page could be isolated from lru between
> > 
> > if (PageLRU(page))
> > and
> > spin_lock_irq(&pgdat->lru_lock);
> 
> yes, it could be a problem. guess the following change could helpful:
> I will update it in new version.

> +       if (lrucare) {
> +               lruvec = lock_page_lruvec_irq(page);
> +               if (likely(PageLRU(page))) {
> +                       ClearPageLRU(page);
> +                       del_page_from_lru_list(page, lruvec, page_lru(page));
> +               } else {
> +                       unlock_page_lruvec_irq(lruvec);
> +                       lruvec = NULL;
> +               }

What about a harder race to hit like a page being on LRU list A when you
look up the lruvec, then it's removed and added to LRU list B by the
time you get the lock?  At that point, you are holding a lock on the
wrong LRU list.  I think you need to check not just that the page
is still PageLRU but also still on the same LRU list.