Re: [PATCH] mm/pgtable/debug: Fix test validating architecture page table helpers

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Christophe Leroy <christophe.leroy@c-s.fr>,
	linux-mm@kvack.org, Mark Rutland <mark.rutland@arm.com>,
	linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	James Hogan <jhogan@kernel.org>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Michal Hocko <mhocko@kernel.org>,
	Dave Hansen <dave.hansen@intel.com>,
	Paul Mackerras <paulus@samba.org>,
	sparclinux@vger.kernel.org,
	Dan Williams <dan.j.williams@intel.com>,
	linux-s390@vger.kernel.org, Jason Gunthorpe <jgg@ziepe.ca>,
	x86@kernel.org, Russell King - ARM Linux <linux@armlinux.org.uk>,
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	Steven Price <Steven.Price@arm.com>,
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	Andrew Morton <akpm@linux-foundation.org>,
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	"David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH] mm/pgtable/debug: Fix test validating architecture page table helpers
Date: Fri, 13 Sep 2019 11:51:35 +0300	[thread overview]
Message-ID: <20190913085135.rfr3zrabghi2qo2t@box> (raw)
In-Reply-To: <bdf7f152-d093-1691-4e96-77da7eb9e20a@arm.com>

On Fri, Sep 13, 2019 at 02:12:45PM +0530, Anshuman Khandual wrote:
> 
> 
> On 09/13/2019 12:41 PM, Christophe Leroy wrote:
> > 
> > 
> > Le 13/09/2019 à 09:03, Christophe Leroy a écrit :
> >>
> >>
> >> Le 13/09/2019 à 08:58, Anshuman Khandual a écrit :
> >>> On 09/13/2019 11:53 AM, Christophe Leroy wrote:
> >>>> Fix build failure on powerpc.
> >>>>
> >>>> Fix preemption imbalance.
> >>>>
> >>>> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
> >>>> ---
> >>>>   mm/arch_pgtable_test.c | 3 +++
> >>>>   1 file changed, 3 insertions(+)
> >>>>
> >>>> diff --git a/mm/arch_pgtable_test.c b/mm/arch_pgtable_test.c
> >>>> index 8b4a92756ad8..f2b3c9ec35fa 100644
> >>>> --- a/mm/arch_pgtable_test.c
> >>>> +++ b/mm/arch_pgtable_test.c
> >>>> @@ -24,6 +24,7 @@
> >>>>   #include <linux/swap.h>
> >>>>   #include <linux/swapops.h>
> >>>>   #include <linux/sched/mm.h>
> >>>> +#include <linux/highmem.h>
> >>>
> >>> This is okay.
> >>>
> >>>>   #include <asm/pgalloc.h>
> >>>>   #include <asm/pgtable.h>
> >>>> @@ -400,6 +401,8 @@ static int __init arch_pgtable_tests_init(void)
> >>>>       p4d_clear_tests(p4dp);
> >>>>       pgd_clear_tests(mm, pgdp);
> >>>> +    pte_unmap(ptep);
> >>>> +
> >>>
> >>> Now the preemption imbalance via pte_alloc_map() path i.e
> >>>
> >>> pte_alloc_map() -> pte_offset_map() -> kmap_atomic()
> >>>
> >>> Is not this very much powerpc 32 specific or this will be applicable
> >>> for all platform which uses kmap_XXX() to map high memory ?
> >>>
> >>
> >> See https://elixir.bootlin.com/linux/v5.3-rc8/source/include/linux/highmem.h#L91
> >>
> >> I think it applies at least to all arches using the generic implementation.
> >>
> >> Applies also to arm:
> >> https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/arm/mm/highmem.c#L52
> >>
> >> Applies also to mips:
> >> https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/mips/mm/highmem.c#L47
> >>
> >> Same on sparc:
> >> https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/sparc/mm/highmem.c#L52
> >>
> >> Same on x86:
> >> https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/x86/mm/highmem_32.c#L34
> >>
> >> I have not checked others, but I guess it is like that for all.
> >>
> > 
> > 
> > Seems like I answered too quickly. All kmap_atomic() do preempt_disable(), but not all pte_alloc_map() call kmap_atomic().
> > 
> > However, for instance ARM does:
> > 
> > https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/arm/include/asm/pgtable.h#L200
> > 
> > And X86 as well:
> > 
> > https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/x86/include/asm/pgtable_32.h#L51
> > 
> > Microblaze also:
> > 
> > https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/microblaze/include/asm/pgtable.h#L495
> 
> All the above platforms checks out to be using k[un]map_atomic(). I am wondering whether
> any of the intermediate levels will have similar problems on any these 32 bit platforms
> or any other platforms which might be using generic k[un]map_atomic().

No. Kernel only allocates pte page table from highmem. All other page
tables are always visible in kernel address space.

-- 
 Kirill A. Shutemov