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SubjectRe: [RFC][PATCH 2/6] pagewalk: only split huge pages when necessary
On Thu, 3 Feb 2011, Dave Hansen wrote:

> > > diff -puN mm/pagewalk.c~pagewalk-dont-always-split-thp mm/pagewalk.c
> > > --- linux-2.6.git/mm/pagewalk.c~pagewalk-dont-always-split-thp 2011-01-27 10:57:02.309914973 -0800
> > > +++ linux-2.6.git-dave/mm/pagewalk.c 2011-01-27 10:57:02.317914965 -0800
> > > @@ -33,19 +33,35 @@ static int walk_pmd_range(pud_t *pud, un
> > >
> > > pmd = pmd_offset(pud, addr);
> > > do {
> > > + again:
> >
> > checkpatch will warn about the indent.
> >
> > > next = pmd_addr_end(addr, end);
> > > - split_huge_page_pmd(walk->mm, pmd);
> > > - if (pmd_none_or_clear_bad(pmd)) {
> > > + if (pmd_none(*pmd)) {
> >
> > Not sure why this has been changed from pmd_none_or_clear_bad(), that's
> > been done even prior to THP.
>
> The bad check will trigger on huge pmds. We can not use it here. We
> can, however, use pmd_none(). The bad check was moved below to where we
> actually dereference the pmd.
>

Ah, right, thanks.

> > > if (walk->pte_hole)
> > > err = walk->pte_hole(addr, next, walk);
> > > if (err)
> > > break;
> > > continue;
> > > }
> > > + /*
> > > + * This implies that each ->pmd_entry() handler
> > > + * needs to know about pmd_trans_huge() pmds
> > > + */
> >
> > Probably needs to be documented somewhere for users of pagewalk?
>
> Probably, but we don't currently have any central documentation for it.
> Guess we could make some, or just ensure that all the users got updated.
> Any ideas where to put it other than the mm_walk struct?
>

I think noting it where struct mm_walk is declared would be best (just a
"/* must handle pmd_trans_huge() */" would be sufficient) although
eventually it might be cleaner to add a ->pmd_huge_entry().


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