Re: [PATCH 1/2] ksm: clean up page_trans_compound_anon_split

[Hugh Dickins <hughd@google.com>]

On Wed, 7 Mar 2012, Andrea Arcangeli wrote:
> On Tue, Mar 06, 2012 at 03:28:43PM -0800, Hugh Dickins wrote:
> > On Thu, 1 Mar 2012, Bob Liu wrote:
> > 
> > > Signed-off-by: Bob Liu <lliubbo@gmail.com>
> > 
> > I agree it looks very much nicer: a patch on these lines would be good.
> > 
> > But you've lost the comment about a return of 1 meaning "Retry later if
> > split_huge_page run from under us", which I think was a helpful comment.
> > 
> > And you've not commented on the functional change which you made:
> > if page_trans_compound_anon() returns NULL, then _split() now returns
> > 1 where before it returned 0.  I suspect that's a reasonable change
> > in a rare case, and better left simple as you have it, than slavishly
> > reproduce the earlier behaviour; but I'd like to have an Ack from the
> > author before we commit your modification.
> 
> Yes, it's not a "noop", I just read the patch through the -mm flow a
> few sec after reading the above.
> 
> > But you didn't Cc Andrea whose code this is, and who understands THP
> > and its races better than anybody: now Cc'ed.
> 
> Thanks for CCing me. Returning 1 when page_trans_compound_anon returns
> NULL, should still be safe, because 1 triggers the bail out path, so
> it won't harm. It should be fully equivalent too because it would bail
> out later in the PageAnon check if page_trans_compound_anon returned 0
> (the function was invoked only on compound pages in the first place).
> 
> So it looks fine.

Thanks, Andrea, that's good.  So Bob, please resubmit with comment on
return value 1 reinstated, and in the commit description explain how
the slight change in operation is benign.

Thanks,
Hugh

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