On Fri, Jan 04, 2013 at 05:03:26AM +0000, Christoffer Dall wrote:
> On Thu, Oct 18, 2012 at 12:15 PM, Steve Capper <steve.capper@xxxxxxx> wrote:
> >
> > /*
> > + * For 3 levels of paging the PTE_EXT_NG bit will be set for user address ptes
> > + * that are written to a page table but not for ptes created with mk_pte.
> > + *
>
> Why is this not the case for 2 levels of paging as well?
>
> Is that because it's always checked against the Linux version, or?
>
>
Yes that's the case, I'll update the comment to reflect that.
> > + * This can cause some comparison tests made by pte_same to fail spuriously and
> > + * lead to other problems.
> > + *
> > + * To correct this behaviour, we mask off PTE_EXT_NG for any pte that is
> > + * present before running the comparison.
>
> nit: This comment doesn't really explain the rationale, I'm assuming
> that pte_same is used to compare only which page gets mapped, assuming
> the attributes etc. remain the same? or also the attributes should be
> the same, only mk_pte sets all of these except the NG bit.
>
I'll expand the comment to include the actual case. Essentially hugetlb_nopage
calls mk_pte to give new_pte and passes this to hugetlb_cow which then performs
a pte_same test against a pte that has already been written out to a page
table; the test fails erroneously due to the mismatch in NG bit.
Unfortunately this then causes a memory leak.
> > + */
> > +#define __HAVE_ARCH_PTE_SAME
> > +static inline int pte_same(pte_t pte_a, pte_t pte_b)
> > +{
> > + pteval_t vala = pte_val(pte_a), valb = pte_val(pte_b);
> > + if (pte_present(pte_a))
> > + vala &= ~L_PTE_CMP_MASKOFF;
> > +
> > + if (pte_present(pte_b))
> > + valb &= ~L_PTE_CMP_MASKOFF;
> > +
> > + return vala == valb;
> > +}
> > +
> > +/*
> > * Encode and decode a swap entry. Swap entries are stored in the Linux
> > * page tables as follows:
> > *
> > --
> > 1.7.9.5
> >
> >
> >
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> > linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
>
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