Re: [PATCH v2 1/1] mm: do not increment pgfault stats when page fault handler retries

Peter Xu
2023-04-18T18:46:24Z

On Tue, Apr 18, 2023 at 09:45:52AM -0700, Suren Baghdasaryan wrote:
> On Tue, Apr 18, 2023 at 8:48 AM Peter Xu <peterx@redhat.com> wrote:
> >
> > On Tue, Apr 18, 2023 at 04:32:27PM +0100, Matthew Wilcox wrote:
> > > ... when we called clone()? A thread by definition has a reference to
> > > its own mm.
> >
> > Ah yes.. thanks!
>
> re: I also had a quick look on do_exit() but I also didn't see where
> do we e.g. wait for all the threads to stop before recycles a mm.
>
> We recycle mm after all refcounts are dropped in the exit path:
> do_exit
> exit_mm
> mmput(if !mm->mm_users)
> mmdrop(if !mm->mm_count)
> free_mm

I assume Matthew means when the task_struct is created with part of
kernel_clone().

copy_mm() has:

if (clone_flags & CLONE_VM) {
mmget(oldmm);
mm = oldmm;
} else {
mm = dup_mm(tsk, current->mm);
if (!mm)
return -ENOMEM;
}

If CLONE_VM, we'll mmget() on the existing mm. If !CLONE_VM, we'll just
create a new one with reference held. For the latter, I think that hides
in mm_init() where it'll just set it to 1:

atomic_set(&mm->mm_users, 1);

With mm_users>0, do_exit() will leave the mm_struct* alone since mmput()
will still be called but not the final step on mmdrop().

Thanks,

--
Peter Xu