Re: [PATCH] rmap 24 pte_young first

[Hugh Dickins]

On Sat, 8 May 2004, Russell King wrote:
> On Sat, May 08, 2004 at 11:39:32PM +0100, Hugh Dickins wrote:
> > On Sat, 8 May 2004, Christoph Hellwig wrote:
> > > 
> > > stupid question - shouldn't the pte_young check simply move to
> > > the beginning of ptep_test_and_clear_young?
> > 
> > I don't think that would be a good idea.  We're used to those
> > test_and_clear operations being atomic, putting an initial non-atomic
> > test inside would make it fundamentally non-atomic.  We know here that
> > it's not the end of the world if we miss a racing transition of the
> > young bit, but it wouldn't be good to hide and force that on others.
> 
> EAGAIN.
> 
> include/asm-generic/pgtable.h:
> 
> #ifndef __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG
> static inline int ptep_test_and_clear_young(pte_t *ptep)
> {
>         pte_t pte = *ptep;
>         if (!pte_young(pte))
>                 return 0;
>         set_pte(ptep, pte_mkold(pte));
>         return 1;
> }
> #endif

Hah!  Delightful refutation of my little lecture.  Thanks a lot for
turning that up.  Hmm.  Well, I guess I need to research that one
further: a first guess would be that the generic variety is silly
to be doing an optimization which the specialist versions don't do:
but perhaps when I look I'll find some of them do.  Can scrub that
patch for now if you prefer, Andrew: world won't stop turning either way.

Hugh

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