On 12/6/20 7:55 PM, Mike Rapoport wrote:
> On Thu, Dec 03, 2020 at 11:23:10PM +0800, carver4lio@xxxxxxx wrote:
>> From: Hailong Liu <liu.hailong6@xxxxxxxxxx>
>>
>> When system in the booting stage, pages span from [start, end] of a memblock
>> are freed to buddy in a order as large as possible (less than MAX_ORDER) at
>> first, then decrease gradually to a proper order(less than end) in a loop.
>>
>> However, *min(MAX_ORDER - 1UL, __ffs(start))* can not get the largest order
>> in some cases.
>
> Do you have examples?
> What is the memory configration that casues suboptimal order selection
> and what is the order in this case?
>
I'm sorry for my careless and inadequate testing(I just test it on my x86
machine with 8 cores).
On my x86_64 machine, the layout of RAM looks like:
/ # cat /proc/iomem
00000100-00000fff : reserved
00001000-0009c7ff : System RAM
0009c800-0009ffff : reserved
.....
100000000-22dffffff : System RAM
22c600000-22d0e01c0 : Kernel code
22d0e01c1-22d96af3f : Kernel data
22dae5000-22dbdcfff : Kernel bss
22e000000-22fffffff : RAM buffer
On my machine, I noticed that when the order of an start pfn in is less than
MAX_ORDER, e.g: the start phy_addr 0x00001000, then the return value *order*
of *min(MAX_ORDER - 1UL, __ffs(start))* will be 1, but the free pages span
of the memblock is more than order 1, it's should be (end - start), I guess.
I tested my ideas with some record code like this:
diff --git a/mm/memblock.c b/mm/memblock.c
index b68ee86788af..b0143e3f75db 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -1928,18 +1928,23 @@ early_param("memblock", early_memblock);
static void __init __free_pages_memory(unsigned long start, unsigned long end)
{
- int order;
+ int order, loop_cnt, adjust_cnt;
+
while (start < end) {
order = min(MAX_ORDER - 1UL, __ffs(start));
- while (start + (1UL << order) > end)
+ while (start + (1UL << order) > end) {
order--;
-
+ adjust_cnt++;
+ }
memblock_free_pages(pfn_to_page(start), start, order);
start += (1UL << order);
+ loop_cnt++;
}
+ pr_info("TST:[start %lu, end %lu]: loop cnt %d, adjust cnt %d\n",
+ loop_cnt++, adjust_cnt++);
}
If I change __ffs(start) to __ffs(end - start), the print info show less
loop_cnt and adjust_cnt on my machine.
>> Instead, *__ffs(end - start)* may be more appropriate and meaningful.
>
> As several people reported using __ffs(end - start) is not correct.
> If the order selection is indeed suboptimal we'd need some better
> formula ;-)
>
>> Signed-off-by: Hailong Liu <liu.hailong6@xxxxxxxxxx>
>> ---
>> mm/memblock.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/memblock.c b/mm/memblock.c
>> index b68ee8678..7c6d0dde7 100644
>> --- a/mm/memblock.c
>> +++ b/mm/memblock.c
>> @@ -1931,7 +1931,7 @@ static void __init __free_pages_memory(unsigned long start, unsigned long end)
>> int order;
>>
>> while (start < end) {
>> - order = min(MAX_ORDER - 1UL, __ffs(start));
>> + order = min(MAX_ORDER - 1UL, __ffs(end - start));
>>
>> while (start + (1UL << order) > end)
>> order--;
>> --
>> 2.17.1
>>
>>
>
