Re: [PATCH v4] mm: per-thread vma caching

[Oleg Nesterov <oleg@redhat.com>]

On 03/09, Linus Torvalds wrote:
>
> On Sun, Mar 9, 2014 at 5:57 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > No, dup_task_struct() is obviously lockless. And the new child is not yet
> > visible to for_each_process_thread().
>
> Ok, then the siimple approach is to just do
>
>     /* Did we miss an invalidate event? *
>     if (mm->seqcount < tsk->seqcount)
>         clear_vma_cache();
>
> after making the new thread visible.
>
> Then the "race" becomes one of "we cannot have 4 billion mmap/munmap
> events in other threads while we're setting up a new thread",

But it's not the "while we're setting up a new thread", it is "since
vmacache_valid() was called list time". And the cloning task can just
sleep(A_LOT) and then do CLONE_VM.

Of course, of course, this race is pute theoretical anyway. But imho
makes sense to fix anyway, and the natural/trivial approach is just to
move vmacache_flush(tsk) from dup_mm() to copy_mm(), right after the
"if (!oldmm)" check.

Oleg.