Re: [PATCH 7/9] mm, page_alloc: move draining pcplists to page isolation users

[Michal Hocko]

On Tue 22-09-20 16:37:10, Vlastimil Babka wrote:
> Currently, pcplists are drained during set_migratetype_isolate() which means
> once per pageblock processed start_isolate_page_range(). This is somewhat
> wasteful. Moreover, the callers might need different guarantees, and the
> draining is currently prone to races and does not guarantee that no page
> from isolated pageblock will end up on the pcplist after the drain.
> 
> Better guarantees are added by later patches and require explicit actions
> by page isolation users that need them. Thus it makes sense to move the
> current imperfect draining to the callers also as a preparation step.
> 
> Suggested-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  mm/memory_hotplug.c | 11 ++++++-----
>  mm/page_alloc.c     |  2 ++
>  mm/page_isolation.c | 10 +++++-----
>  3 files changed, 13 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 9db80ee29caa..08f729922e18 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1524,6 +1524,8 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages)
>  		goto failed_removal;
>  	}
>  
> +	drain_all_pages(zone);
> +
>  	arg.start_pfn = start_pfn;
>  	arg.nr_pages = nr_pages;
>  	node_states_check_changes_offline(nr_pages, zone, &arg);
> @@ -1574,11 +1576,10 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages)
>  		}
>  
>  		/*
> -		 * per-cpu pages are drained in start_isolate_page_range, but if
> -		 * there are still pages that are not free, make sure that we
> -		 * drain again, because when we isolated range we might
> -		 * have raced with another thread that was adding pages to pcp
> -		 * list.
> +		 * per-cpu pages are drained after start_isolate_page_range, but
> +		 * if there are still pages that are not free, make sure that we
> +		 * drain again, because when we isolated range we might have
> +		 * raced with another thread that was adding pages to pcp list.
>  		 *
>  		 * Forward progress should be still guaranteed because
>  		 * pages on the pcp list can only belong to MOVABLE_ZONE
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 901907799bdc..4e37bc3f6077 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8432,6 +8432,8 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>  	if (ret)
>  		return ret;
>  
> +	drain_all_pages(cc.zone);
> +
>  	/*
>  	 * In case of -EBUSY, we'd like to know which page causes problem.
>  	 * So, just fall through. test_pages_isolated() has a tracepoint
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index abfe26ad59fd..57d8bc1e7760 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -49,7 +49,6 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
>  
>  		__mod_zone_freepage_state(zone, -nr_pages, mt);
>  		spin_unlock_irqrestore(&zone->lock, flags);
> -		drain_all_pages(zone);
>  		return 0;
>  	}
>  
> @@ -167,11 +166,12 @@ __first_valid_page(unsigned long pfn, unsigned long nr_pages)
>   *
>   * Please note that there is no strong synchronization with the page allocator
>   * either. Pages might be freed while their page blocks are marked ISOLATED.
> - * In some cases pages might still end up on pcp lists and that would allow
> + * A call to drain_all_pages() after isolation can flush most of them. However
> + * in some cases pages might still end up on pcp lists and that would allow
>   * for their allocation even when they are in fact isolated already. Depending
> - * on how strong of a guarantee the caller needs drain_all_pages might be needed
> - * (e.g. __offline_pages will need to call it after check for isolated range for
> - * a next retry).
> + * on how strong of a guarantee the caller needs, further drain_all_pages()
> + * might be needed (e.g. __offline_pages will need to call it after check for
> + * isolated range for a next retry).
>   *
>   * Return: 0 on success and -EBUSY if any part of range cannot be isolated.
>   */
> -- 
> 2.28.0

-- 
Michal Hocko
SUSE Labs