Re: [PATCH v20 08/20] mm: page_idle_get_page() does not need lru_lock

Matthew Wilcox
2020-11-05T15:44:24Z

On Thu, Nov 05, 2020 at 10:36:49AM -0500, Johannes Weiner wrote:
> But the code is highly specific - synchronizing one struct page member
> for one particular use case. Let's keep at least a reference to what
> we are synchronizing against. There is a non-zero chance that if the
> comment goes out of date, so does the code. How about this?
>
> /*
> * page_idle does a lockless/optimistic rmap scan on page->mapping.
> * Make sure the compiler doesn't split the stores of anon_vma and
> * the PAGE_MAPPING_ANON type identifier, otherwise the rmap code
> * could mistake the mapping for a struct address_space and crash.
> */

Fine by me! There may be other cases where seeing a split store would
be bad, so I didn't want to call out page_idle explicitly. But if you
want to, I'm happy with this comment.