linux-kernel - Re: [PATCH 09/25] mm, compaction: Use the page allocator bulk-free helper for lists of pages

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Date:   Wed, 16 Jan 2019 09:46:39 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Linux-MM <linux-mm@...ck.org>,
        David Rientjes <rientjes@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>, 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 09/25] mm, compaction: Use the page allocator bulk-free
 helper for lists of pages

On Tue, Jan 15, 2019 at 01:39:28PM +0100, Vlastimil Babka wrote:
> On 1/4/19 1:49 PM, 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. The performance impact is
> > limited but it should reduce lock contention slightly in some cases.
> > The main benefit is removing some partially duplicated code.
> > 
> > Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> 
> ...
> 
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -2876,18 +2876,26 @@ 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));
> 
> I've thought about it again and still think it can cause spurious
> warnings. We enter this function with one page pin, which means somebody
> else might be doing pfn scanning and get_page_unless_zero() with
> success, so there are two pins. Then we do the put_page_testzero() above
> and go back to one pin, and warn. You said "this function simply does
> not expect it and the callers do not violate the rule", but this is
> rather about potential parallel pfn scanning activity and not about this
> function's callers. Maybe there really is no parallel pfn scanner that
> would try to pin a page with a state the page has when it's processed by
> this function, but I wouldn't bet on it (any state checks preceding the
> pin might also be racy etc.).
> 

Ok, I'll drop this patch because in theory you're right. I wouldn't think
that parallel PFN scanning is likely to trigger it but gup is a potential
issue. While this also will increase CPU usage slightly again, it'll be
no worse than it was before and again, I don't want to stall the entire
series over a relatively small optimisation.

Thanks Vlastimil!

-- 
Mel Gorman
SUSE Labs