Messages in this thread | | | Date | Fri, 12 Jun 2020 16:47:44 +0200 | From | Christian Brauner <> | Subject | Re: vdso_join_timens() question |
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On Thu, Jun 11, 2020 at 08:17:12PM +0100, Dmitry Safonov wrote: > Hi Christian, > > On 6/11/20 12:02 PM, Christian Brauner wrote: > > Hey, > > > > I'm about to finish a patch to add CLONE_NEWTIME support to setns(). > > Since setns() now allows to attach to a multiple namespaces at the same > > time I've also reworked it to be atomic (already upstream). Either all > > namespaces are switched or no namespace is switched. All namespaces > > basically now have a commit mode after which installation should ideally > > not fail anymore. That could work for CLONE_NEWTIME too, I think. The > > only blocker to this is vdso_join_timens() which can fail due to > > mmap_write_lock_killable(). > > > > Is it possible to change this to mmap_write_lock()? So sm like: > > > > diff --git a/arch/x86/entry/vdso/vma.c b/arch/x86/entry/vdso/vma.c > > index ea7c1f0b79df..5c5b4cc61fce 100644 > > --- a/arch/x86/entry/vdso/vma.c > > +++ b/arch/x86/entry/vdso/vma.c > > @@ -144,8 +144,7 @@ int vdso_join_timens(struct task_struct *task, struct time_namespace *ns) > > struct mm_struct *mm = task->mm; > > struct vm_area_struct *vma; > > > > - if (mmap_write_lock_killable(mm)) > > - return -EINTR; > > + mmap_write_lock(mm); > > > > for (vma = mm->mmap; vma; vma = vma->vm_next) { > > unsigned long size = vma->vm_end - vma->vm_start; > > I think, it should be fine.
Great! :)
> > I'm thinking if it actually could be downgraded to mmap_read_lock().. > Probably, write lock was being over-cautious.
Let's use mmap_write_lock() for now and revisit this later, I guess.
> > > vdso_join_timens() is called in two places. Once during fork() and once > > during timens_install(). I would only need the mmap_write_lock() change > > for the latter. So alternatively we could have: > > > > __vdso_join_timens_unlocked() > > > > and then have/expose: > > > > vdso_join_timens_fork() > > { > > if (mmap_write_lock_killable(mm)) > > return -EINTR; > > __vdso_join_timens_unlocked() > > mmap_write_unlock(mm); > > } > > > > and > > > > vdso_join_timens_install() > > { > > mmap_write_lock(mm); > > __vdso_join_timens_unlocked() > > mmap_write_unlock(mm); > > } > > I think it's not needed. On fork() it's called on creation of new timens: > > : if (nsproxy->time_ns == nsproxy->time_ns_for_children) > : return 0; > > So the vdso_join_timens() is called on setns() or on creation of new > namespace, which both are quite heavy operations themselves (in sense of > locking).
Ok, great. It'll like be the week after next until I can send the patchset for setns(<nsfd>/<pidfd>, CLONE_NEWTIME) out.
Thanks! Christian
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