linux-kernel - Re: [PATCH 2/3] mm: thp: Fix the update_mmu_cache() last argument passing in mm/huge

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Date:	Wed, 19 Sep 2012 10:12:28 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Ralf Baechle <ralf@...ux-mips.org>,
	Will Deacon <will.deacon@....com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	mhocko@...e.cz, Steve Capper <steve.capper@....com>
Subject: Re: [PATCH 2/3] mm: thp: Fix the update_mmu_cache() last argument
 passing in mm/huge_memory.c

On 18 September 2012 20:33, Andrew Morton <akpm@...ux-foundation.org> wrote:
> On Sat, 15 Sep 2012 15:38:33 +0200
> Ralf Baechle <ralf@...ux-mips.org> wrote:
>> On Tue, Sep 11, 2012 at 05:47:15PM +0100, Will Deacon wrote:
>> > The update_mmu_cache() takes a pointer (to pte_t by default) as the last
>> > argument but the huge_memory.c passes a pmd_t value. The patch changes
>> > the argument to the pmd_t * pointer.
>> >
>> > Signed-off-by: Catalin Marinas <catalin.marinas@....com>
>> > Signed-off-by: Steve Capper <steve.capper@....com>
>> > Signed-off-by: Will Deacon <will.deacon@....com>
>> > ---
>> >  mm/huge_memory.c |    6 +++---
>> >  1 files changed, 3 insertions(+), 3 deletions(-)
>> >
>> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> > index 57c4b93..4aa6d02 100644
>> > --- a/mm/huge_memory.c
>> > +++ b/mm/huge_memory.c
>> > @@ -934,7 +934,7 @@ int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
>> >             entry = pmd_mkyoung(orig_pmd);
>> >             entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
>> >             if (pmdp_set_access_flags(vma, haddr, pmd, entry,  1))
>> > -                   update_mmu_cache(vma, address, entry);
>> > +                   update_mmu_cache(vma, address, pmd);
>>
>> Documentation/cachetlb.txt will need an update as well.  Currently it says:
>>
>> 5) void update_mmu_cache(struct vm_area_struct *vma,
>>                          unsigned long address, pte_t *ptep)
>
> Yes please.

Should we just use a generic (void *) for the last argument or force a
cast in mm/huge_memory.c?

Ralf's point is that transparent huge page code calls update_mmu_cache
with a (pmd_t *) as the last argument. This could make sense for THP
as it assumes that huge pages can only be created at the pmd level.
But that's unlike mm/hugetlb.c which casts huge page types to pte_t,
even though on ARM they are implemented at the pmd level.

On ARM (with VIPT caches) update_mmu_cache() is empty like on x86,
though a static inline rather than macro.

-- 
Catalin
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