On Fri, Sep 25, 2020 at 02:06:59PM -0700, Linus Torvalds wrote: > On Fri, Sep 25, 2020 at 12:56 PM Linus Torvalds > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > And honestly, since this is all getting fairly late in the rc, and it > > took longer than I thought, I think we should do the GFP_ATOMIC > > approach for now - not great, but since it only triggers for this case > > that really should never happen anyway, I think it's probably the best > > thing for 5.9, and we can improve on things later. > > I'm not super-happy with this patch, but I'm throwing it out anyway, in case > > (a) somebody can test it - I don't have any test cases It looks like it will work and resolve the RDMA case that triggered this discussion. I will send it to our testers, should hear back around Monday. They previously said Peter's v1 patch worked, expecting the same here, unless something unexpected hits the extra pre-conditions. Though, we might hit the THP case and find it fails... > (b) somebody can find issues and improve on it The THP hunks from Peter's series looked pretty straightforward, I'd include at least the PMD one. As a tiny optimization, the preconditions in copy_normal_page() could order the atomics last to try and reduce the atomics done per fork. > I'm happy to take Peter's series too, this is more of an alternative > simplified version to keep the discussion going. I don't completely grok the consequences of the anon_vma check. We can exclude file backed mappings as they are broken for pinning anyhow, so what is left that could be MAP_PRIVATE of a non-anon_vma? Feels obscure, probably OK. If something does break userspace could use MAP_SHARED and be fixed. Otherwise, I do prefer Peter's version because of the GFP_KERNEL. To touch on your other email.. It was my hope we could move away from the "This should never happen". From a RDMA POV this idea was sort of managable years ago, but now I have folks writing data science/ML software in Python that deep under the libraries use RDMA and has pinned pages. It was a Python program that detected this regression. Having all that "just work" regardless of what foolish stuff happens in the Python layer is very appealing. Jason
