Le 12/03/2024 à 17:53, Pasha Tatashin a écrit :
> On Tue, Mar 12, 2024 at 3:16 AM Nikolay Borisov <nik.borisov@xxxxxxxx> wrote:
>>
>>
>>
>> On 11.03.24 г. 18:46 ч., Pasha Tatashin wrote:
>>> In preporation for dynamic kernel stacks do not zero the whole span of
>>> the stack, but instead only the pages that are part of the vm_area.
>>>
>>> This is because with dynamic stacks we might have only partially
>>> populated stacks.
>>>
>>> Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
>>> ---
>>> kernel/fork.c | 6 ++++--
>>> 1 file changed, 4 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/kernel/fork.c b/kernel/fork.c
>>> index 6a2f2c85e09f..41e0baee79d2 100644
>>> --- a/kernel/fork.c
>>> +++ b/kernel/fork.c
>>> @@ -263,8 +263,8 @@ static int memcg_charge_kernel_stack(struct vm_struct *vm)
>>> static int alloc_thread_stack_node(struct task_struct *tsk, int node)
>>> {
>>> struct vm_struct *vm_area;
>>> + int i, j, nr_pages;
>>> void *stack;
>>> - int i;
>>>
>>> for (i = 0; i < NR_CACHED_STACKS; i++) {
>>> vm_area = this_cpu_xchg(cached_stacks[i], NULL);
>>> @@ -282,7 +282,9 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
>>> stack = kasan_reset_tag(vm_area->addr);
>>>
>>> /* Clear stale pointers from reused stack. */
>>> - memset(stack, 0, THREAD_SIZE);
>>> + nr_pages = vm_area->nr_pages;
>>> + for (j = 0; j < nr_pages; j++)
>>> + clear_page(page_address(vm_area->pages[j]));
>>
>> Can't this be memset(stack, 0, nr_pages*PAGE_SIZE) ?
>
> No, we can't, because the pages can be physically discontiguous.
>
But the pages were already physically discontiguous before your change,
what's the difference ?
It doesn't matter that the pages are physically discontiguous as far as
they are virtually contiguous, which should still be the case here for a
stack.
Nevertheless, from powerpc point of view I'm happy with clear_page()
which is more optimised than memset(0)
Christophe
