[PATCH v2 1/2] arm64: vmemmap: use virtual projection of linear region

[Ard Biesheuvel]

On 8 March 2016 at 09:15, Ard Biesheuvel <ard.biesheuvel at linaro.org> wrote:
>
>
>> On 8 mrt. 2016, at 08:07, David Daney <ddaney.cavm at gmail.com> wrote:
>>
>>> On 02/26/2016 08:57 AM, Ard Biesheuvel wrote:
>>> Commit dd006da21646 ("arm64: mm: increase VA range of identity map") made
>>> some changes to the memory mapping code to allow physical memory to reside
>>> at an offset that exceeds the size of the virtual mapping.
>>>
>>> However, since the size of the vmemmap area is proportional to the size of
>>> the VA area, but it is populated relative to the physical space, we may
>>> end up with the struct page array being mapped outside of the vmemmap
>>> region. For instance, on my Seattle A0 box, I can see the following output
>>> in the dmesg log.
>>>
>>>    vmemmap : 0xffffffbdc0000000 - 0xffffffbfc0000000   (     8 GB maximum)
>>>              0xffffffbfc0000000 - 0xffffffbfd0000000   (   256 MB actual)
>>>
>>> We can fix this by deciding that the vmemmap region is not a projection of
>>> the physical space, but of the virtual space above PAGE_OFFSET, i.e., the
>>> linear region. This way, we are guaranteed that the vmemmap region is of
>>> sufficient size, and we can even reduce the size by half.
>>>
>>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>>
>> I see this commit now in Linus' kernel.org tree in v4.5-rc7.
>>
>> FYI:  I am seeing a crash that goes away when I revert this.  My kernel has some other modifications (our NUMA patches) so I haven't yet fully tracked this down on an unmodified kernel, but this is what I am getting:
>>
>

I managed to reproduce and diagnose this. The problem is that vmemmap
is no longer zone aligned, which causes trouble in the zone based
rounding that occurs in memory_present. The below patch fixes this by
rounding down the subtracted offset. Since this implies that the
region could stick off the other end, it also reverts the halving of
the region size.


--------8<----------
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index f50608674580..ed57c0865290 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -40,7 +40,7 @@
  * VMALLOC_END: extends to the available space below vmmemmap, PCI I/O space,
  *     fixed mappings and modules
  */
-#define VMEMMAP_SIZE           ALIGN((1UL << (VA_BITS - PAGE_SHIFT -
1)) * sizeof(struct page), PUD_SIZE)
+#define VMEMMAP_SIZE           ALIGN((1UL << (VA_BITS - PAGE_SHIFT))
* sizeof(struct page), PUD_SIZE)

 #ifndef CONFIG_KASAN
 #define VMALLOC_START          (VA_START)
@@ -52,7 +52,8 @@
 #define VMALLOC_END            (PAGE_OFFSET - PUD_SIZE - VMEMMAP_SIZE - SZ_64K)

 #define VMEMMAP_START          (VMALLOC_END + SZ_64K)
-#define vmemmap                        ((struct page *)VMEMMAP_START
- (memstart_addr >> PAGE_SHIFT))
+#define vmemmap                        ((struct page *)VMEMMAP_START - \
+                               ((memstart_addr >> PAGE_SHIFT) &
PAGE_SECTION_MASK))

 #define FIRST_USER_ADDRESS     0UL