On Jul 20, 2022, at 1:10 PM, Axel Rasmussen <axelrasmussen@xxxxxxxxxx> wrote: > On Wed, Jul 20, 2022 at 10:42 AM Nadav Amit <namit@xxxxxxxxxx> wrote: >> On Jul 19, 2022, at 7:32 PM, Peter Xu <peterx@xxxxxxxxxx> wrote: >> >>> ⚠ External Email >>> >>> On Tue, Jul 19, 2022 at 11:55:21PM +0000, Nadav Amit wrote: >>>> Anyhow, I do want to clarify a bit about the “cross-process support” >>>> userfaultfd situation. Basically, you can already get cross-process support >>>> today, by using calling userfaultfd() on the controlled process and calling >>>> pidfd_open() from another process. It does work and I do not remember any >>>> issues that it introduced (in contrast, for instance, to io-uring, that >>>> would break if you use userfaultfd+iouring+fork today). >>> >>> Do you mean to base it on pidof_getfd()? >> >> autocorrect? :) >> >> I did refer to pidfd_getfd() as a syscall that can be used today by one >> process to control the address space of another process. I did not intend to >> use it for the actual implementation. >> >>> Just want to mention that this will still need collaboration of the target >>> process as userfaultfd needs to be created explicitly there. From that POV >>> it's still more similar to general SCM_RIGHTS trick to pass over the fd but >>> just to pass it in a different way. >> >> There are also some tricks you can do with ptrace in order not to need the >> collaboration, but they are admittedly fragile. >> >>> IMHO the core change about having /proc/pid/userfaultfd is skipping that >>> only last step to create the handle. >> >> Yes. The point that I was trying to make is that there are no open issues >> with adding support for remote process control through >> /proc/pid/userfaultfd. This is in contrast, for example, for using io-uring >> with userfaultfd. For instance, if you try to use io-uring TODAY with >> userfaultfd (without the async support that I need to add), and you try to >> monitor the fork event, things would break (the new userfaultfd file >> descriptor after fork would be installed on the io-worker thread). >> >> This is all to say that it is really simple to add support for one process >> monitoring userfaultfd of another process, since I understood that Axel had >> concerned that this might be utterly broken… > > Mostly I was worried it would be nontrivial to implement, and it isn't > a use case I plan to use so I was hoping to ignore it and defer it to > some future patches. ;) > > But, if it "just works" I'm happy to include it in v5. There is a problem though, since for many use-cases you do need process_madvisev(MADV_DONTNEED) which is unsupported, and you also need - in some use-cases - to be able to skip pinned pages. These are patches that I still need to send. So I leave it to you to make up your mind whether it is reasonable to add it now without this support.
