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

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