Re: [PATCH v6 1/5] powerpc/mm: Implement set_memory() routines

[Christophe Leroy <christophe.leroy@c-s.fr>]

Le 03/02/2020 à 01:46, Russell Currey a écrit :
> On Wed, 2020-01-08 at 13:52 +0100, Christophe Leroy wrote:
>>
>> Le 24/12/2019 à 06:55, Russell Currey a écrit :
>>> diff --git a/arch/powerpc/mm/Makefile b/arch/powerpc/mm/Makefile
>>> index 5e147986400d..d0a0bcbc9289 100644
>>> --- a/arch/powerpc/mm/Makefile
>>> +++ b/arch/powerpc/mm/Makefile
>>> @@ -20,3 +20,4 @@ obj-$(CONFIG_HIGHMEM)		+= highmem.o
>>>    obj-$(CONFIG_PPC_COPRO_BASE)	+= copro_fault.o
>>>    obj-$(CONFIG_PPC_PTDUMP)	+= ptdump/
>>>    obj-$(CONFIG_KASAN)		+= kasan/
>>> +obj-$(CONFIG_ARCH_HAS_SET_MEMORY) += pageattr.o
>>
>> CONFIG_ARCH_HAS_SET_MEMORY is set inconditionnally, I think you
>> should
>> add pageattr.o to obj-y instead. CONFIG_ARCH_HAS_XXX are almost
>> never
>> used in Makefiles
> 
> Fair enough, will keep that in mind

I forgot I commented that. I'll do it in v3.

>>> +	pte_t pte_val;
>>> +
>>> +	// invalidate the PTE so it's safe to modify
>>> +	pte_val = ptep_get_and_clear(&init_mm, addr, ptep);
>>> +	flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
>>
>> Why flush a range for a single page ? On most targets this will do a
>> tlbia which is heavy, while a tlbie would suffice.
>>
>> I think flush_tlb_kernel_range() should be replaced by something
>> flushing only a single page.
> 
> You might be able to help me out here, I wanted to do that but the only
> functions I could find that flushed single pages needed a
> vm_area_struct, which I can't get.

I sent out two patches for that, one for book3s/32 and one for nohash:
https://patchwork.ozlabs.org/patch/1231983/
https://patchwork.ozlabs.org/patch/1232223/

Maybe one for book3s/64 would be needed as well ? Can you do it if needed ?


> 
>>
>>> +
>>> +	// modify the PTE bits as desired, then apply
>>> +	switch (action) {
>>> +	case SET_MEMORY_RO:
>>> +		pte_val = pte_wrprotect(pte_val);
>>> +		break;
>>> +	case SET_MEMORY_RW:
>>> +		pte_val = pte_mkwrite(pte_val);
>>> +		break;
>>> +	case SET_MEMORY_NX:
>>> +		pte_val = pte_exprotect(pte_val);
>>> +		break;
>>> +	case SET_MEMORY_X:
>>> +		pte_val = pte_mkexec(pte_val);
>>> +		break;
>>> +	default:
>>> +		WARN_ON(true);
>>> +		return -EINVAL;
>>
>> Is it worth checking that the action is valid for each page ? I
>> think
>> validity of action should be checked in change_memory_attr(). All
>> other
>> functions are static so you know they won't be called from outside.
>>
>> Once done, you can squash __change_page_attr() into
>> change_page_attr(),
>> remove the ret var and return 0 all the time.
> 
> Makes sense to fold things into a single function, but in terms of
> performance it shouldn't make a difference, right?  I still have to
> check the action to determine what to change (unless I replace passing
> SET_MEMORY_RO into apply_to_page_range() with a function pointer to
> pte_wrprotect() for example).

pte_wrprotect() is a static inline.

> 
>>
>>> +	}
>>> +
>>> +	set_pte_at(&init_mm, addr, ptep, pte_val);
>>> +
>>> +	return 0;
>>> +}
>>> +
>>> +static int change_page_attr(pte_t *ptep, unsigned long addr, void
>>> *data)
>>> +{
>>> +	int ret;
>>> +
>>> +	spin_lock(&init_mm.page_table_lock);
>>> +	ret = __change_page_attr(ptep, addr, data);
>>> +	spin_unlock(&init_mm.page_table_lock);
>>> +
>>> +	return ret;
>>> +}
>>> +
>>> +int change_memory_attr(unsigned long addr, int numpages, int
>>> action)
>>> +{
>>> +	unsigned long start = ALIGN_DOWN(addr, PAGE_SIZE);
>>> +	unsigned long size = numpages * PAGE_SIZE;
>>> +
>>> +	if (!numpages)
>>> +		return 0;
>>> +
>>> +	return apply_to_page_range(&init_mm, start, size,
>>> change_page_attr, &action);
>>
>> Use (void*)action instead of &action (see upper comment)
> 
> To get this to work I had to use (void *)(size_t)action to stop the
> compiler from complaining about casting an int to a void*, is there a
> better way to go about it?  Works fine, just looks gross.

Yes, use long instead (see my v3)

Christophe