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

[Mel Gorman <mgorman@techsingularity.net>]

On Tue, Dec 18, 2018 at 10:55:31AM +0100, Vlastimil Babka wrote:
> 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@techsingularity.net>
> 
> Acked-by: Vlastimil Babka <vbabka@suse.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...
> 

While there are cases where such a pattern is legal, this function
simply does not expect it and the callers do not violate the rule. If it
ever gets a new user that makes mistakes, they'll get the warning. Sure,
the page leaks but it'll be in a state where it's unsafe to do anything
else with it.

-- 
Mel Gorman
SUSE Labs