On Fri, Sep 13, 2019 at 02:12:45PM +0530, Anshuman Khandual wrote:

On 09/13/2019 12:41 PM, Christophe Leroy wrote:

quoted

Le 13/09/2019 à 09:03, Christophe Leroy a écrit :

quoted

Le 13/09/2019 à 08:58, Anshuman Khandual a écrit :

quoted

On 09/13/2019 11:53 AM, Christophe Leroy wrote:

quoted

Fix build failure on powerpc.

Fix preemption imbalance.

Signed-off-by: Christophe Leroy <redacted>
---
  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.

quoted

  #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