Re: [PATCH] mm, memory_hotplug: teach has_unmovable_pages about of LRU migrateable pages

[Baoquan He]

On 11/05/18 at 10:28am, Michal Hocko wrote:
> On Mon 05-11-18 10:14:07, Michal Hocko wrote:
> > Maybe we can add a retry for movable zone pages.
> 
> Or something like this. Ugly as hell, no question about that. I also
> have to think about this some more to convince myself this will not
> result in an endless loop under some situations.

Testing, will tell the result.

> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 48ceda313332..342d66eca0f3 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7779,12 +7779,16 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count,
>  	pfn = page_to_pfn(page);
>  	for (found = 0, iter = 0; iter < pageblock_nr_pages; iter++) {
>  		unsigned long check = pfn + iter;
> +		unsigned long saved_flags;
>  
>  		if (!pfn_valid_within(check))
>  			continue;
>  
>  		page = pfn_to_page(check);
>  
> +retry:
> +		saved_flags = READ_ONCE(page->flags);
> +
>  		if (PageReserved(page))
>  			goto unmovable;
>  
> @@ -7840,6 +7844,13 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count,
>  				page->mapping->a_ops->migratepage)
>  			continue;
>  
> +		/*
> +		 * We might race with the allocation of the page so retry
> +		 * if flags have changed.
> +		 */
> +		if (saved_flags != READ_ONCE(page->flags))
> +			goto retry;
> +
>  		/*
>  		 * If there are RECLAIMABLE pages, we need to check
>  		 * it.  But now, memory offline itself doesn't call
> -- 
> Michal Hocko
> SUSE Labs