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

[Linus Torvalds]

On Sat, Jan 9, 2021 at 5:19 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> And no, I didn't make the UFFDIO_WRITEPROTECT code take the mmap_sem
> for writing. For whoever wants to look at that, it's
> mwriteprotect_range() in mm/userfaultfd.c and the fix is literally to
> turn the read-lock (and unlock) into a write-lock (and unlock).

Oh, and if it wasn't obvious, we'll have to debate what to do with
trying to mprotect() a pinned page. Do we just ignore the pinned page
(the way my clear_refs patch did)? Or do we turn it into -EBUSY? Or
what?

So it's not *just* the locking that needs to be fixed. But just take a
look at that suggested clear_refs patch of mine - it sure isn't
complicated.

              Linus