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

[Andrea Arcangeli]

Hello,

On Sat, Jan 09, 2021 at 07:44:35PM -0500, Andrea Arcangeli wrote:
> allowing a child to corrupt memory in the parent. That's a problem
> that could happen not-maliciously too. So the scenario described

I updated the above partly quoted sentence since in the previous
version it didn't have full accuracy:

https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git/commit/?id=fc5a76b1c14e5e6cdc64ece306fc03773662d98a

"However since a single transient GUP pin on a tail page, would elevate
the page_count for all other tail pages (unlike the mapcount which is
subpage granular), the COW page reuse inaccuracy would then cross
different vmas and the effect would happen at a distance in vma of
different processes. A single GUP pin taken on a subpage mapped in a
different process could trigger 511 false positive COWs copies in the
local process, after a fork()."

This a best effort to try to document all side effects, but it'd be
great to hear from Kirill too on the above detail to have
confirmation.

Thanks and have a great weekend everyone,
Andrea