linux-kernel - Re: [PATCH v6 20/37] mm: fix non-compound multi-order memory accounting in __free

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Date: Thu, 21 Mar 2024 17:04:05 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Suren Baghdasaryan <surenb@...gle.com>
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Subject: Re: [PATCH v6 20/37] mm: fix non-compound multi-order memory
 accounting in __free_pages

On Thu, Mar 21, 2024 at 04:48:53PM +0000, Matthew Wilcox wrote:
> On Thu, Mar 21, 2024 at 09:36:42AM -0700, Suren Baghdasaryan wrote:
> > +++ b/mm/page_alloc.c
> > @@ -4700,12 +4700,15 @@ void __free_pages(struct page *page, unsigned int order)
> >  {
> >  	/* get PageHead before we drop reference */
> >  	int head = PageHead(page);
> > +	struct alloc_tag *tag = pgalloc_tag_get(page);
> >  
> >  	if (put_page_testzero(page))
> >  		free_the_page(page, order);
> > -	else if (!head)
> > +	else if (!head) {
> > +		pgalloc_tag_sub_pages(tag, (1 << order) - 1);
> >  		while (order-- > 0)
> >  			free_the_page(page + (1 << order), order);
> > +	}
> 
> Why do you need these new functions instead of just:
> 
> +	else if (!head) {
> +		pgalloc_tag_sub(page, (1 << order) - 1);
> 		while (order-- > 0)
> 			free_the_page(page + (1 << order), order);
> +	}

Actually, I'm not sure this is safe (I don't fully understand codetags,
so it may be safe).  What can happen is that the put_page() can come in
before the pgalloc_tag_sub(), and then that page can be allocated again.  
Will that cause confusion?