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

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Date:   Fri, 13 Sep 2019 11:51:35 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Christophe Leroy <christophe.leroy@....fr>, linux-mm@...ck.org,
        Mark Rutland <mark.rutland@....com>,
        linux-ia64@...r.kernel.org, linux-sh@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        James Hogan <jhogan@...nel.org>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Michal Hocko <mhocko@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Paul Mackerras <paulus@...ba.org>, sparclinux@...r.kernel.org,
        Dan Williams <dan.j.williams@...el.com>,
        linux-s390@...r.kernel.org, Jason Gunthorpe <jgg@...pe.ca>,
        x86@...nel.org, Russell King - ARM Linux <linux@...linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>,
        Steven Price <Steven.Price@....com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Vlastimil Babka <vbabka@...e.cz>,
        linux-snps-arc@...ts.infradead.org,
        Kees Cook <keescook@...omium.org>,
        Mark Brown <broonie@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        linux-arm-kernel@...ts.infradead.org,
        Sri Krishna chowdary <schowdary@...dia.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-mips@...r.kernel.org, Ralf Baechle <ralf@...ux-mips.org>,
        linux-kernel@...r.kernel.org, Paul Burton <paul.burton@...s.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Vineet Gupta <vgupta@...opsys.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev@...ts.ozlabs.org,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] mm/pgtable/debug: Fix test validating architecture page
 table helpers

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@....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