Re: Recurring warning in page_copy_sane (inside copy_page_to_iter) when running stress tests involving drop_caches

[Matthew Wilcox]

On Wed, May 15, 2019 at 08:02:17AM -0700, Eric Dumazet wrote:
> On Wed, May 15, 2019 at 7:43 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > You're seeing a race between page_address(page) being called twice.
> > Between those two calls, something has caused the page to be removed from
> > the page_address_map() list.  Eric's patch avoids calling page_address(),
> > so apply it and be happy.
> 
> Hmm... wont the kmap_atomic() done later, after page_copy_sane() would
> suffer from the race ?
> 
> It seems there is a real bug somewhere to fix.

No.  page_address() called before the kmap_atomic() will look through
the list of mappings and see if that page is mapped somewhere.  We unmap
lazily, so all it takes to trigger this race is that the page _has_
been mapped before, and its mapping gets torn down during this call.

While the page is kmapped, its mapping cannot be torn down.