Re: [v6 01/15] x86/mm: reserve only exiting low pages

Pasha Tatashin
2017-08-17T15:38:31Z

Hi Michal,

While working on a bug that was reported to me by "kernel test robot".

unable to handle kernel NULL pointer dereference at (null)

The issue was that page_to_pfn() on that configuration was looking for a
section inside flags fields in "struct page". So, reserved but
unavailable memory should have its "struct page" zeroed.

Therefore, I am going to remove this patch from my series, but instead
have a new patch that iterates through:

reserved && !memory memblocks, and zeroes struct pages for them. Since
for that memory struct pages will never go through __init_single_page(),
yet some fields might still be accessed.

Pasha

On 08/14/2017 09:55 AM, Michal Hocko wrote:
> Let's CC Hpa on this one. I am still not sure it is correct. The full
> series is here
> http://lkml.kernel.org/r/1502138329-123460-1-git-send-email-pasha.tatashin@oracle.com
>
> On Mon 07-08-17 16:38:35, Pavel Tatashin wrote:
>> Struct pages are initialized by going through __init_single_page(). Since
>> the existing physical memory in memblock is represented in memblock.memory
>> list, struct page for every page from this list goes through
>> __init_single_page().
>>
>> The second memblock list: memblock.reserved, manages the allocated memory.
>> The memory that won't be available to kernel allocator. So, every page from
>> this list goes through reserve_bootmem_region(), where certain struct page
>> fields are set, the assumption being that the struct pages have been
>> initialized beforehand.
>>
>> In trim_low_memory_range() we unconditionally reserve memoryfrom PFN 0, but
>> memblock.memory might start at a later PFN. For example, in QEMU,
>> e820__memblock_setup() can use PFN 1 as the first PFN in memblock.memory,
>> so PFN 0 is not on memblock.memory (and hence isn't initialized via
>> __init_single_page) but is on memblock.reserved (and hence we set fields in
>> the uninitialized struct page).
>>
>> Currently, the struct page memory is always zeroed during allocation,
>> which prevents this problem from being detected. But, if some asserts
>> provided by CONFIG_DEBUG_VM_PGFLAGS are tighten, this problem may become
>> visible in existing kernels.
>>
>> In this patchset we will stop zeroing struct page memory during allocation.
>> Therefore, this bug must be fixed in order to avoid random assert failures
>> caused by CONFIG_DEBUG_VM_PGFLAGS triggers.
>>
>> The fix is to reserve memory from the first existing PFN.
>>
>> Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
>> Reviewed-by: Steven Sistare <steven.sistare@oracle.com>
>> Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>
>> Reviewed-by: Bob Picco <bob.picco@oracle.com>
>> ---
>> arch/x86/kernel/setup.c | 5 ++++-
>> 1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
>> index 3486d0498800..489cdc141bcb 100644
>> --- a/arch/x86/kernel/setup.c
>> +++ b/arch/x86/kernel/setup.c
>> @@ -790,7 +790,10 @@ early_param("reservelow", parse_reservelow);
>>
>> static void __init trim_low_memory_range(void)
>> {
>> - memblock_reserve(0, ALIGN(reserve_low, PAGE_SIZE));
>> + unsigned long min_pfn = find_min_pfn_with_active_regions();
>> + phys_addr_t base = min_pfn << PAGE_SHIFT;
>> +
>> + memblock_reserve(base, ALIGN(reserve_low, PAGE_SIZE));
>> }
>>
>> /*
>> --
>> 2.14.0
>