linux-kernel - Re: [PATCH 08/14] mm, compaction: Use the page allocator bulk-free helper for lists of pages

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Date:   Tue, 18 Dec 2018 10:55:31 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Linux-MM <linux-mm@...ck.org>
Cc:     David Rientjes <rientjes@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>, ying.huang@...el.com,
        kirill@...temov.name, Andrew Morton <akpm@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 08/14] mm, compaction: Use the page allocator bulk-free
 helper for lists of pages

On 12/15/18 12:03 AM, Mel Gorman wrote:
> release_pages() is a simpler version of free_unref_page_list() but it
> tracks the highest PFN for caching the restart point of the compaction
> free scanner. This patch optionally tracks the highest PFN in the core
> helper and converts compaction to use it.
> 
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

Nit below:

> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2961,18 +2961,26 @@ void free_unref_page(struct page *page)
>  /*
>   * Free a list of 0-order pages
>   */
> -void free_unref_page_list(struct list_head *list)
> +void __free_page_list(struct list_head *list, bool dropref,
> +				unsigned long *highest_pfn)
>  {
>  	struct page *page, *next;
>  	unsigned long flags, pfn;
>  	int batch_count = 0;
>  
> +	if (highest_pfn)
> +		*highest_pfn = 0;
> +
>  	/* Prepare pages for freeing */
>  	list_for_each_entry_safe(page, next, list, lru) {
> +		if (dropref)
> +			WARN_ON_ONCE(!put_page_testzero(page));

That will warn just once, but then page will remain with elevated count
and free_unref_page_prepare() will warn either immediately or later
depending on DEBUG_VM, for each page.
Also IIRC it's legal for basically anyone to do get_page_unless_zero()
and later put_page(), and this would now cause warning. Maybe just test
for put_page_testzero() result without warning, and continue? Hm but
then we should still do a list_del() and that becomes racy after
dropping our ref...

>  		pfn = page_to_pfn(page);
>  		if (!free_unref_page_prepare(page, pfn))
>  			list_del(&page->lru);
>  		set_page_private(page, pfn);
> +		if (highest_pfn && pfn > *highest_pfn)
> +			*highest_pfn = pfn;
>  	}
>  
>  	local_irq_save(flags);
>