On Sat, Jan 09, 2021 at 09:54:05PM -0500, Andrea Arcangeli wrote: > 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. Yes, this side effect is possible. But I wouldn't worry about too much. If it routinely happens in a real workloads (I doubt it does), the workload can tune it with MADV_NOHUGEPAGE/MADV_DONTFORK/MADV_WIPEONFORK or something. -- Kirill A. Shutemov
