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SubjectRe: vdso_join_timens() question
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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