Re: [PATCH v2] arm64: mm: fix pmd_leaf()

[Will Deacon <will@kernel.org>]

On Wed, Apr 13, 2022 at 11:39:49AM +0100, Steven Price wrote:
> On 13/04/2022 11:19, Will Deacon wrote:
> > On Mon, Apr 11, 2022 at 08:26:53PM +0800, Muchun Song wrote:
> >> The pmd_leaf() is used to test a leaf mapped PMD, however, it misses
> >> the PROT_NONE mapped PMD on arm64.  Fix it.  A real world issue [1]
> >> caused by this was reported by Qian Cai.
> >>
> >> Link: https://patchwork.kernel.org/comment/24798260/ [1]
> >> Fixes: 8aa82df3c123 ("arm64: mm: add p?d_leaf() definitions")
> >> Reported-by: Qian Cai <quic_qiancai@quicinc.com>
> >> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> >> ---
> >> v2:
> >> - Replace pmd_present() with pmd_val() since we expect pmd_leaf() works
> >>   well on non-present pmd case.
> >>
> >>  arch/arm64/include/asm/pgtable.h | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> >> index ad9b221963d4..00cdd2d895d3 100644
> >> --- a/arch/arm64/include/asm/pgtable.h
> >> +++ b/arch/arm64/include/asm/pgtable.h
> >> @@ -551,7 +551,7 @@ extern pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
> >>  				 PMD_TYPE_TABLE)
> >>  #define pmd_sect(pmd)		((pmd_val(pmd) & PMD_TYPE_MASK) == \
> >>  				 PMD_TYPE_SECT)
> >> -#define pmd_leaf(pmd)		pmd_sect(pmd)
> >> +#define pmd_leaf(pmd)		(pmd_val(pmd) && !(pmd_val(pmd) & PMD_TABLE_BIT))
> >>  #define pmd_bad(pmd)		(!pmd_table(pmd))
> > 
> > I'm still trying to get my head around the desired semantics here.
> > 
> > If we want to fix the original report, then we need to take PROT_NONE
> > entries into account. The easiest way to do that is, as you originally
> > suggested, by using pmd_present():
> > 
> > #define pmd_leaf(pmd)	(pmd_present(pmd) && !pmd_table(pmd))
> > 
> > But now you seem to be saying that !pmd_present() entries should also be
> > considered as pmd_leaf() -- is there a real need for that?
> > 
> > If so, then I think this simply becomes:
> > 
> > #define pmd_leaf(pmd)	(!pmd_table(pmd))
> > 
> > which is, amusingly, identical to pmd_bad().
> > 
> > The documentation/comment that Steven referred to also desperately needs
> > clarifying as it currently states:
> > 
> >   "Only meaningful when called on a valid entry."
> > 
> > whatever that means.
> 
> The intention at the time is that this had the same meaning as
> pmd_huge() (when CONFIG_HUGETLB_PAGE is defined), which would then match
> this patch. This is referred in the comment, albeit in a rather weak way:
> 
> >  * This differs from p?d_huge() by the fact that they are always available (if
> >  * the architecture supports large pages at the appropriate level) even
> >  * if CONFIG_HUGETLB_PAGE is not defined.
> 
> However, the real issue here is that the definition of pmd_leaf() isn't
> clear. I know what the original uses of it needed but since then it's
> been used in other areas, and I'm afraid my 'documentation' isn't
> precise enough to actually be useful.
> 
> At the time I wrote that comment I think I meant "valid" in the AArch64
> sense (i.e. the LSB of the entry). PROT_NONE isn't 'valid' by that
> definition (and I hadn't considered it). But of course that definition
> of 'valid' is pretty meaningless in the cross-architecture case.

arm64 'valid' + PROT_NONE is roughly what 'present' means. So we could say
that this only works for present entries, but then Muchun's latest patch
wants to work with !present which is why I tried to work this through.

Will

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