Re: [PATCH 4/5] mm: Do early cow for pinned pages during fork() for ptes

[Peter Xu <peterx@redhat.com>]

On Tue, Sep 22, 2020 at 08:44:00PM +0200, Oleg Nesterov wrote:
> On 09/22, Peter Xu wrote:
> >
> > Or I can also do it in inverted order if you think better:
> >
> >         if (unlikely(copy_ret == COPY_MM_BREAK_COW)) {
> >                 WARN_ON_ONCE(!data.cow_new_page);
> >                 ...
> >         }
> 
> Peter, let me say this again. I don't understand this code enough, you
> can safely ignore me ;)

Why? I appreciate every single comment from you! :)

> 
> However. Personally I strongly prefer the above. Personally I really
> dislike this part of 4/5:
> 
> 	 again:
> 	+	/* We don't reset this for COPY_MM_BREAK_COW */
> 	+	memset(&data, 0, sizeof(data));
> 	+
> 	+again_break_cow:
> 
> If we rely on "copy_ret == COPY_MM_BREAK_COW" we can unify "again" and
> "again_break_cow", we don't need to clear ->cow_new_page, this makes the
> logic more understandable. To me at least ;)

I see your point.  I'll definitely try it out.  I think I'll at least use what
you preferred above since it's actually the same as before, logically.  Then
I'll consider drop the again_break_cow, as long as I'm still as confident after
I do the change on not leaking anything :).

Thanks,

-- 
Peter Xu