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SubjectRe: [PATCH] page_alloc: Fix freeing non-compound pages
On Thu, Sep 24, 2020 at 11:00:02AM +0200, peterz@infradead.org wrote:
> 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?

That's more complicated. It looks like this:

Fix this by converting P0 into a compound page if it is not freed by
__free_pages().

Fixes: e286781d5f2e ("mm: speculative page references")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index fab5e97dc9ca..3e9f6e6694e7 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4943,10 +4943,25 @@ static inline void free_the_page(struct page *page, unsigned int order)
__free_pages_ok(page, order);
}

+/*
+ * Have to be careful when freeing a non-compound allocation in case somebody
+ * else takes a temporary reference on the first page and then calls put_page()
+ */
void __free_pages(struct page *page, unsigned int order)
{
- if (put_page_testzero(page))
- free_the_page(page, order);
+ if (likely(page_ref_freeze(page, 1)))
+ goto free;
+ if (likely(order == 0 || PageHead(page))) {
+ if (put_page_testzero(page))
+ goto free;
+ return;
+ }
+
+ prep_compound_page(page, order);
+ put_page(page);
+ return;
+free:
+ free_the_page(page, order);
}
EXPORT_SYMBOL(__free_pages);
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