Re: [PATCH 0/1] mm: restore full accuracy in COW page reuse

[Matthew Wilcox]

On Mon, Jan 11, 2021 at 02:18:13PM -0800, Linus Torvalds wrote:
> The whole "optimistic page references throigh page cache" etc are
> complete non-issues, because the whole point is that we already know
> it's not a page cache page. There is simply no other way to reach that
> page than through GUP.

The thing about the speculative page cache references is that they can
temporarily bump a refcount on a page which _used_ to be in the page
cache and has now been reallocated as some other kind of page.

Now, this is obviously rare, so if it's only a performance question,
it'll be fine.  If there's a correctness issue with copying pages that
would otherwise not have been copied, then it's a problem.