Re: [PATCH v2 1/3] vmalloc: Choose a better start address in vm_area_register_early()

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2021/8/1 23:23, Catalin Marinas wrote:
> On Tue, Jul 20, 2021 at 10:51:03AM +0800, Kefeng Wang wrote:
>> There are some fixed locations in the vmalloc area be reserved
>> in ARM(see iotable_init()) and ARM64(see map_kernel()), but for
>> pcpu_page_first_chunk(), it calls vm_area_register_early() and
>> choose VMALLOC_START as the start address of vmap area which
>> could be conflicted with above address, then could trigger a
>> BUG_ON in vm_area_add_early().
>>
>> Let's choose the end of existing address range in vmlist as the
>> start address instead of VMALLOC_START to avoid the BUG_ON.
>>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>> ---
>>   mm/vmalloc.c | 8 +++++---
>>   1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index d5cd52805149..a98cf97f032f 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -2238,12 +2238,14 @@ void __init vm_area_add_early(struct vm_struct *vm)
>>    */
>>   void __init vm_area_register_early(struct vm_struct *vm, size_t align)
>>   {
>> -	static size_t vm_init_off __initdata;
>> +	unsigned long vm_start = VMALLOC_START;
>> +	struct vm_struct *tmp;
>>   	unsigned long addr;
>>   
>> -	addr = ALIGN(VMALLOC_START + vm_init_off, align);
>> -	vm_init_off = PFN_ALIGN(addr + vm->size) - VMALLOC_START;
>> +	for (tmp = vmlist; tmp; tmp = tmp->next)
>> +		vm_start = (unsigned long)tmp->addr + tmp->size;
>>   
>> +	addr = ALIGN(vm_start, align);
>>   	vm->addr = (void *)addr;
>>   
>>   	vm_area_add_early(vm);
> Is there a risk of breaking other architectures? It doesn't look like to
> me but I thought I'd ask.

Before this patch, vm_init_off is to record the offset from VMALLOC_START,

but it use VMALLOC_START as start address on the function 
vm_area_register_early()

called firstly,  this will cause the BUG_ON.

With this patch, the most important change is that we choose the start 
address via

dynamic calculate the 'start' address by traversing the list.

[wkf@localhost linux-next]$ git grep vm_area_register_early
arch/alpha/mm/init.c: vm_area_register_early(&console_remap_vm, PAGE_SIZE);
arch/x86/xen/p2m.c:     vm_area_register_early(&vm, PMD_SIZE * 
PMDS_PER_MID_PAGE);
mm/percpu.c:    vm_area_register_early(&vm, PAGE_SIZE);
[wkf@localhost linux-next]$ git grep vm_area_add_early
arch/arm/mm/ioremap.c:  vm_area_add_early(vm);
arch/arm64/mm/mmu.c:    vm_area_add_early(vma);

x86/alpha won't call vm_area_add_early(), only arm64 could call both vm_area_add_early()
and  vm_area_register_early() when this patchset is merged. so it won't break other architectures.

>
> Also, instead of always picking the end, could we search for a range
> that fits?
We only need a space in vmalloc range,  using end or a range in the 
middle is not different.
>