> On Dec 21, 2020, at 1:24 PM, Yu Zhao <yuzhao@xxxxxxxxxx> wrote: > > On Mon, Dec 21, 2020 at 12:26:22PM -0800, Linus Torvalds wrote: >> On Mon, Dec 21, 2020 at 12:23 PM Nadav Amit <nadav.amit@xxxxxxxxx> wrote: >>> Using mmap_write_lock() was my initial fix and there was a strong pushback >>> on this approach due to its potential impact on performance. >> >> From whom? >> >> Somebody who doesn't understand that correctness is more important >> than performance? And that userfaultfd is not the most important part >> of the system? >> >> The fact is, userfaultfd is CLEARLY BUGGY. >> >> Linus > > Fair enough. > > Nadav, for your patch (you might want to update the commit message). Yes, the commit message is completely off. Will fix. Thanks for your guidance and assistance. > > Reviewed-by: Yu Zhao <yuzhao@xxxxxxxxxx> > > While we are all here, there is also clear_soft_dirty() that could > use a similar fix… Let me try to build a small reproducer for clear_soft_dirty() and then I’ll send another patch for it too. BTW: In general, I think that you are right, and that changing of PTEs should not require taking mmap_lock for write. However, I am not sure cow_user_page() is not the only one that poses a problem and whether a more systematic solution is needed. If cow_user_pages() is the only problem, do you think it is possible to do the copying while holding the PTL? It works for normal-pages, but I am not sure whether special-pages pose special problems. Anyhow, this is an enhancement that we can try later. Thanks again, Nadav
