On Mon, Dec 9, 2024 at 10:28 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > > On 07.12.24 09:29, Mateusz Guzik wrote: > > Explicitly pre-checking the count adds nothing as atomic_add_unless > > starts with doing the same thing. iow no functional changes. > > I recall that we added that check because with the hugetlb vmemmap > optimization, some of the tail pages we don't ever expect to be modified > (because they are fake-duplicated) might be mapped R/O. > > If the arch implementation of atomic_add_unless() would trigger an > unconditional write fault, we'd be in trouble. That would likely only be > the case if the arch provides a dedicate instruction. > > atomic_add_unless()->raw_atomic_add_unless() > > Nobody currently defines arch_atomic_add_unless(). > > raw_atomic_fetch_add_unless()->arch_atomic_fetch_add_unless() is defined > on some architectures. > > I scanned some of the inline-asm, and I think most of them perform a > check first. > Huh. Some arch triggering a write fault despite not changing the value is not something I thought about. Sounds pretty broken to me if any arch was to do it, but then stranger things did happen. However, if this is seen as a real concern, then I think the best way forward is to drop the patch (and maybe instead add a comment what's up with the extra load). -- Mateusz Guzik <mjguzik gmail.com>
