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SubjectRe: [PATCH] page_alloc: Fix freeing non-compound pages
On Tue, Sep 22, 2020 at 03:00:17PM +0100, Matthew Wilcox (Oracle) wrote:
> Here is a very rare race which leaks memory:
>
> 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
> Frees P0
> P0 merged with 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)
> P0 has refcount 1
> put_page(P0)
> P1 is not freed
>
> 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.
>
> Fixes: e286781d5f2e ("mm: speculative page references")
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
> mm/page_alloc.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index fab5e97dc9ca..5db74797db39 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4943,10 +4943,19 @@ static inline void free_the_page(struct page *page, unsigned int order)
> __free_pages_ok(page, order);
> }
>
> +/*
> + * If we free a non-compound allocation, another thread may have a
> + * speculative reference to the first page. It has no way of knowing
> + * about the rest of the allocation, so we have to free all but the
> + * first page here.
> + */
> 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);
> }
> EXPORT_SYMBOL(__free_pages);

So the obvious question I have here is why not teach put_page() to free
the whole thing?

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