> On Dec 17, 2021, at 3:29 PM, David Hildenbrand <david@xxxxxxxxxx> wrote: > > On 17.12.21 23:58, Linus Torvalds wrote: >> >> And you can't even do the optimistic case without taking the lock, >> because in your world, a COW that optimistically copies in the case of >> a race condition is fundamentally *wrong* and buggy. Because in your >> world-view, GUP and COW are very different and have different rules, >> but you need things to be *exact*, and they aren’t. I understand the discussion mainly revolves correctness, which is obviously the most important property, but I would like to mention that having transient get_page() calls causing unnecessary COWs can cause hard-to-analyze and hard-to-avoid performance degradation. COW means a page copy, a TLB flush and potentially a TLB shootdown, which is the most painful, specifically on VMs. So I think that any solution should be able to limit the cases/number of unnecessary COW operations to be minimal.
