On Mon, Sep 14, 2020 at 10:32:11AM -0700, Linus Torvalds wrote: > On Mon, Sep 14, 2020 at 7:38 AM Jason Gunthorpe <jgg@xxxxxxxxxx> wrote: > > > > I don't have a detailed explanation right now, but this patch appears > > to be causing a regression where RDMA subsystem tests fail. Tests > > return to normal when this patch is reverted. > > > > It kind of looks like the process is not seeing DMA'd data to a > > pin_user_pages()? > > I'm a nincompoop. I actually _talked_ to Hugh Dickins about this when > he raised concerns, and I dismissed his concerns with "but PAGE_PIN is > special". > > As usual, Hugh was right. Page pinning certainly _is_ special, but > it's not that different from the regular GUP code. > > But in the meantime, I have a lovely confirmation from the kernel test > robot, saying that commit 09854ba94c results in a > "vm-scalability.throughput 31.4% improvement", which was what I was > hoping for - the complexity wasn't just complexity, it was active > badness due to the page locking horrors. > > I think what we want to do is basically do the "early COW", but only > do it for FOLL_PIN (and not turn them into writes for anything but the > COW code). So basically redo the "enforced COW mechanism", but rather > than do it for everything, now do it only for FOLL_PIN, and only in > that COW path. > > Peter - any chance you can look at this? I'm still looking at the page > lock fairness performance regression, although I now think I have a > test patch for Phoronix to test out. Sure, I'll try to prepare something like that and share it shortly. Thanks, -- Peter Xu
