On Tue, Jun 21, 2022 at 10:23:32AM +0200, David Hildenbrand wrote: > On 17.06.22 03:41, Peter Xu wrote: > > We have had FAULT_FLAG_INTERRUPTIBLE but it was never applied to GUPs. One > > issue with it is that not all GUP paths are able to handle signal delivers > > besides SIGKILL. > > > > That's not ideal for the GUP users who are actually able to handle these > > cases, like KVM. > > > > KVM uses GUP extensively on faulting guest pages, during which we've got > > existing infrastructures to retry a page fault at a later time. Allowing > > the GUP to be interrupted by generic signals can make KVM related threads > > to be more responsive. For examples: > > > > (1) SIGUSR1: which QEMU/KVM uses to deliver an inter-process IPI, > > e.g. when the admin issues a vm_stop QMP command, SIGUSR1 can be > > generated to kick the vcpus out of kernel context immediately, > > > > (2) SIGINT: which can be used with interactive hypervisor users to stop a > > virtual machine with Ctrl-C without any delays/hangs, > > > > (3) SIGTRAP: which grants GDB capability even during page faults that are > > stuck for a long time. > > > > Normally hypervisor will be able to receive these signals properly, but not > > if we're stuck in a GUP for a long time for whatever reason. It happens > > easily with a stucked postcopy migration when e.g. a network temp failure > > happens, then some vcpu threads can hang death waiting for the pages. With > > the new FOLL_INTERRUPTIBLE, we can allow GUP users like KVM to selectively > > enable the ability to trap these signals. > > This makes sense to me. I assume relevant callers will detect "GUP > failed" but also "well, there is a signal to handle" and cleanly back > off, correct? Correct, via an -EINTR. One thing to mention is that the gup user behavior will be the same as before if the caller didn't explicilty pass in FOLL_INTERRUPTIBLE with the gup call. So after the whole series applied only kvm (and only some path of kvm, not all GUP; I only touched up the x86 slow page fault path) to handle this, but that'll be far enough to cover 99.99% use cases that I wanted to take care of. E.g., some kvm request to gup on some guest apic page may not still be able to respond to a SIGUSR1 but that's very very rare, and we can always add more users of FOLL_INTERRUPTIBLE when the code is ready to benefit from the fast respondings. Thanks, -- Peter Xu
