Re: [PATCH 6/6] ksm: unstable_tree_search_insert error checking cleanup

[Andrea Arcangeli <aarcange@redhat.com>]

On Sun, Oct 25, 2015 at 04:18:05PM -0700, Hugh Dickins wrote:
> On Thu, 15 Oct 2015, Andrea Arcangeli wrote:
> 
> > get_mergeable_page() can only return NULL (in case of errors) or the
> > pinned mergeable page. It can't return an error different than
> > NULL. This makes it more readable and less confusion in addition to
> > optimizing the check.
> > 
> > Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> 
> I share your sentiment, prefer to avoid an unnecessary IS_ERR_OR_NULL.
> And you may be right that it's unnecessary; but that's far from clear
> to me, and you haven't changed the IS_ERR_OR_NULL after follow_page()
> in get_mergeable_page() where it originates, so I wonder if you just
> got confused on this.
> 
> Even if you have established that there's currently no way that
> follow_page(vma, addr, FOLL_GET) could return an -errno on a vma
> validated by find_mergeable_vma(), I think we'd still be better off
> to allow for some future -errno there; but I'd be happy for you to
> keep the change below, but also adjust get_mergeable_page() to
> convert an -errno immediately to NULL after follow_page().
> 
> So, I think this is gently Nacked in its present form,
> but a replacement eagerly Acked.

The "out:" label is followed by page = NULL, so if follow_page returns
an error, get_mergeable_page still cannot return an error.

If this wasn't the case, get_mergeable_page would return an
uninitialized pointer if find_mergeable_vma would return NULL.

I guess the IS_ERR_OR_NULL that I removed, was the direct result of
overlooking the location of the "out:".

If there was a return after the "out:" the readability would have been
improved, but I haven't touched the code around the "out:". That's the
way it was before. Now I'll add a return in this same patch after the
"out:" before resubmitting without 1/6.

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