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

[Christophe Leroy <christophe.leroy@xxxxxx>]

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@xxxxxx>
> > > ---
> > >   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

Christophe