Re: [PATCH] mm: page_alloc: fix memcg accounting leak in speculative cache lookup

Matthew Wilcox
2021-03-26T02:53:44Z

On Thu, Mar 25, 2021 at 06:55:42PM -0700, Hugh Dickins wrote:
> The first reason occurred to me this morning. I thought I had been
> clever to spot the PageHead race which you fix here. But now I just feel
> very stupid not to have spotted the very similar memcg_data race. The
> speculative racer may call mem_cgroup_uncharge() from __put_single_page(),
> and the new call to split_page_memcg() do nothing because page_memcg(head)
> is already NULL.
>
> And is it even safe there, to sprinkle memcg_data through all of those
> order-0 subpages, when free_the_page() is about to be applied to a
> series of descending orders? I could easily be wrong, but I think
> free_pages_prepare()'s check_free_page() will find that is not
> page_expected_state().

So back to something more like my original patch then?

+++ b/mm/page_alloc.c
@@ -5081,9 +5081,15 @@ 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);
+ else if (!PageHead(page)) {
+ while (order-- > 0) {
+ struct page *tail = page + (1 << order);
+#ifdef CONFIG_MEMCG
+ tail->memcg_data = page->memcg_data;
+#endif
+ free_the_page(tail, order);
+ }
+ }
}
EXPORT_SYMBOL(__free_pages);

We can cache page->memcg_data before calling put_page_testzero(),
just like we cache the Head flag in Johannes' patch.

> But, after all that, I'm now thinking that Matthew's original
> e320d3012d25 ("mm/page_alloc.c: fix freeing non-compound pages")
> is safer reverted. The put_page_testzero() in __free_pages() was
> not introduced for speculative pagecache: it was there in 2.4.0,
> and atomic_dec_and_test() in 2.2, I don't have older trees to hand.

I think you're confused in that last assertion. According to
linux-fullhistory, the first introduction of __free_pages was 2.3.29pre3
(September 1999), where it did indeed use put_page_testzero:

+extern inline void __free_pages(struct page *page, unsigned long order)
+{
+ if (!put_page_testzero(page))
+ return;
+ __free_pages_ok(page, order);
+}

Before that, we had only free_pages() and __free_page().

> So, it has "always" been accepted that multiple references to a
> high-order non-compound page can be given out and released: maybe
> they were all released with __free_pages() of the right order, or
> maybe only the last had to get that right; but as __free_pages()
> stands today, all but the last caller frees all but the first
> subpage. A very rare leak seems much safer.
>
> I don't have the answer (find somewhere in struct page to squirrel
> away the order, even when it's a non-compound page?), and I think
> each of us would much rather be thinking about other things at the
> moment. But for now it looks to me like NAK to this patch, and
> revert of e320d3012d25.

We did discuss that possibility prior to the introduction of
e320d3012d25. Here's one such: