Re: [RFC PATCH v3 03/12] mm: memcontrol: make lruvec lock safe when the LRU pages reparented

[Roman Gushchin]

On Wed, Apr 21, 2021 at 03:00:50PM +0800, Muchun Song wrote:
> The diagram below shows how to make the page lruvec lock safe when the
> LRU pages reparented.
> 
> lock_page_lruvec(page)
>     retry:
>         lruvec = mem_cgroup_page_lruvec(page);
> 
>         // The page is reparented at this time.
>         spin_lock(&lruvec->lru_lock);
> 
>         if (unlikely(lruvec_memcg(lruvec) != page_memcg(page)))
>             // Acquired the wrong lruvec lock and need to retry.
>             // Because this page is on the parent memcg lruvec list.
>             goto retry;
> 
>         // If we reach here, it means that page_memcg(page) is stable.
> 
> memcg_reparent_objcgs(memcg)
>     // lruvec belongs to memcg and lruvec_parent belongs to parent memcg.
>     spin_lock(&lruvec->lru_lock);
>     spin_lock(&lruvec_parent->lru_lock);
> 
>     // Move all the pages from the lruvec list to the parent lruvec list.
> 
>     spin_unlock(&lruvec_parent->lru_lock);
>     spin_unlock(&lruvec->lru_lock);
> 
> After we acquire the lruvec lock, we need to check whether the page is
> reparented. If so, we need to reacquire the new lruvec lock. On the
> routine of the LRU pages reparenting, we will also acquire the lruvec
> lock (Will be implemented in the later patch). So page_memcg() cannot
> be changed when we hold the lruvec lock.
> 
> Since lruvec_memcg(lruvec) is always equal to page_memcg(page) after
> we hold the lruvec lock, lruvec_memcg_debug() check is pointless. So
> remove it.
> 
> This is a preparation for reparenting the LRU pages.
> 
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>

Acked-by: Roman Gushchin <guro@xxxxxx>

Thanks!