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    SubjectRe: [PATCH] mm/pgtable/debug: Fix test validating architecture page table helpers
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    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(). There can be many
    permutations here.

    p4dp = p4d_alloc(mm, pgdp, vaddr);
    pudp = pud_alloc(mm, p4dp, vaddr);
    pmdp = pmd_alloc(mm, pudp, vaddr);

    Otherwise pte_alloc_map()/pte_unmap() looks good enough which will atleast take care of
    a known failure.

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