Re: [PATCH v2] page_alloc: Fix freeing non-compound pages

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 26 Sep 2020 22:39:19 +0100 "Matthew Wilcox (Oracle)" <willy@infradead.org> wrote:

> Here is a very rare race which leaks memory:

Not worth a cc:stable?

> Page P0 is allocated to the page cache.  Page P1 is free.
> 
> Thread A                Thread B                Thread C
> find_get_entry():
> xas_load() returns P0
> 						Removes P0 from page cache
> 						P0 finds its buddy P1
> 			alloc_pages(GFP_KERNEL, 1) returns P0
> 			P0 has refcount 1
> page_cache_get_speculative(P0)
> P0 has refcount 2
> 			__free_pages(P0)

			__free_pages(P0, 1), I assume.

> 			P0 has refcount 1
> put_page(P0)

but this is implicitly order 0

> P1 is not freed

huh.

> Fix this by freeing all the pages in __free_pages() that won't be freed
> by the call to put_page().  It's usually not a good idea to split a page,
> but this is a very unlikely scenario.
> 
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4947,6 +4947,9 @@ void __free_pages(struct page *page, unsigned int order)
>  {
>  	if (put_page_testzero(page))
>  		free_the_page(page, order);
> +	else if (!PageHead(page))
> +		while (order-- > 0)
> +			free_the_page(page + (1 << order), order);

Well that's weird and scary looking.  `page' has non-zero refcount yet
we go and free random followon pages.  Methinks it merits an
explanatory comment?