[PATCH v5 3/3] arm64: hibernate: Support DEBUG_PAGEALLOC

[james.morse@arm.com (James Morse)]

Hi Catalin

On 22/08/16 19:51, Catalin Marinas wrote:
> On Mon, Aug 22, 2016 at 06:35:19PM +0100, James Morse wrote:
>> diff --git a/arch/arm64/kernel/hibernate.c b/arch/arm64/kernel/hibernate.c
>> index b4082017c4cb..da4470de1807 100644
>> --- a/arch/arm64/kernel/hibernate.c
>> +++ b/arch/arm64/kernel/hibernate.c
>> @@ -235,6 +235,7 @@ out:
>>  	return rc;
>>  }
>>  
>> +#define dcache_clean_range(start, end)	__flush_dcache_area(start, (end - start))
>>  
>>  int swsusp_arch_suspend(void)
>>  {
>> @@ -252,8 +253,14 @@ int swsusp_arch_suspend(void)
>>  	if (__cpu_suspend_enter(&state)) {
>>  		ret = swsusp_save();
>>  	} else {
>> -		/* Clean kernel to PoC for secondary core startup */
>> -		__flush_dcache_area(LMADDR(KERNEL_START), KERNEL_END - KERNEL_START);
>> +		/* Clean kernel core startup/idle code to PoC*/
>> +		dcache_clean_range(__mmuoff_text_start, __mmuoff_text_end);
>> +		dcache_clean_range(__mmuoff_data_start, __mmuoff_data_end);
>> +		dcache_clean_range(__idmap_text_start, __idmap_text_end);
>> +
>> +		/* Clean kvm setup code to PoC? */
>> +		if (el2_reset_needed())
>> +			dcache_clean_range(__hyp_idmap_text_start, __hyp_idmap_text_end);
>>  
>>  		/*
>>  		 * Tell the hibernation core that we've just restored
>> @@ -269,6 +276,32 @@ int swsusp_arch_suspend(void)
>>  	return ret;
>>  }
>>  
>> +static void _copy_pte(pte_t *dst_pte, pte_t *src_pte, unsigned long addr)
>> +{
>> +	unsigned long pfn = virt_to_pfn(addr);
> 
> I assume this is only called on the kernel linear mapping (according to
> the copy_page_tables() use in swsusp_arch_resume). Otherwise
> virt_to_pfn() would not work.

Yes, hibernate only save/restores the linear map, on the assumption that
contains everything.


> Something I missed in the original hibernation support but it may look
> better if you have something like:
> 
> 	pte_t pte = *src_pte;

Sure,


>> +
>> +	if (pte_valid(*src_pte)) {
>> +		/*
>> +		 * Resume will overwrite areas that may be marked
>> +		 * read only (code, rodata). Clear the RDONLY bit from
>> +		 * the temporary mappings we use during restore.
>> +		 */
>> +		set_pte(dst_pte, __pte(pte_val(*src_pte) & ~PTE_RDONLY));
> 
> and here:
> 
> 		set_pte(dst_pte, pte_mkwrite(pte));

pte_write() doesn't clear the PTE_RDONLY flag.
Should it be changed?


>> +	} else if (debug_pagealloc_enabled()) {
>> +		/*
>> +		 * debug_pagealloc may have removed the PTE_VALID bit if
>> +		 * the page isn't in use by the resume kernel. It may have
>> +		 * been in use by the original kernel, in which case we need
>> +		 * to put it back in our copy to do the restore.
>> +		 *
>> +		 * Check for mappable memory that gives us a translation
>> +		 * like part of the linear map.
>> +		 */
>> +		if (pfn_valid(pfn) && pte_pfn(*src_pte) == pfn)
> 
> Is there a case where this condition is false?

Hopefully not, but I tried to avoid marking whatever happens to be there as
valid. This is as paranoid as I can make it: checking the pfn should be mapped,
and that the output-address part of the record is correct.

If you're happy with the assumption that only valid records ever appear in the
linear map page tables, (and that anything marked not-valid is a result of
debug_pagealloc), then we can change this to !pte_none().


>> +			set_pte(dst_pte, __pte((pte_val(*src_pte) & ~PTE_RDONLY) | PTE_VALID));
> 
> With some more macros:
> 
> 			set_pte(dst_pte, pte_mkwrite(pte_mkpresent(pte)))
> 
> (pte_mkpresent() needs to be added)

>> +	}
>> +}
>> +
>>  static int copy_pte(pmd_t *dst_pmd, pmd_t *src_pmd, unsigned long start,
>>  		    unsigned long end)
>>  {
>> @@ -284,13 +317,7 @@ static int copy_pte(pmd_t *dst_pmd, pmd_t *src_pmd, unsigned long start,
>>  
>>  	src_pte = pte_offset_kernel(src_pmd, start);
>>  	do {
>> -		if (!pte_none(*src_pte))
> 
> You seem to no longer check for pte_none(). Is this not needed or
> covered by the pte_pfn() != pfn check above?

A bit of both:
Previously this copied over any values it found. _copy_pte() now copies valid
values, and if debug_pagealloc is turned on, tries to guess whether the
non-valid values should be copied and marked valid.


> 
>> -			/*
>> -			 * Resume will overwrite areas that may be marked
>> -			 * read only (code, rodata). Clear the RDONLY bit from
>> -			 * the temporary mappings we use during restore.
>> -			 */
>> -			set_pte(dst_pte, __pte(pte_val(*src_pte) & ~PTE_RDONLY));
>> +		_copy_pte(dst_pte, src_pte, addr);
>>  	} while (dst_pte++, src_pte++, addr += PAGE_SIZE, addr != end);
>>  
>>  	return 0;
>> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
>> index ca6d268e3313..b6c0da84258c 100644
>> --- a/arch/arm64/mm/pageattr.c
>> +++ b/arch/arm64/mm/pageattr.c
>> @@ -139,4 +139,42 @@ void __kernel_map_pages(struct page *page, int numpages, int enable)
>>  					__pgprot(0),
>>  					__pgprot(PTE_VALID));
>>  }
>> -#endif
>> +#ifdef CONFIG_HIBERNATION
>> +/*
>> + * When built with CONFIG_DEBUG_PAGEALLOC and CONFIG_HIBERNATION, this function
>> + * is used to determine if a linear map page has been marked as not-present by
>> + * CONFIG_DEBUG_PAGEALLOC. Walk the page table and check the PTE_VALID bit.
>> + * This is based on kern_addr_valid(), which almost does what we need.
>> + */
>> +bool kernel_page_present(struct page *page)
>> +{
>> +	pgd_t *pgd;
>> +	pud_t *pud;
>> +	pmd_t *pmd;
>> +	pte_t *pte;
>> +	unsigned long addr = (unsigned long)page_address(page);
>> +
>> +	pgd = pgd_offset_k(addr);
>> +	if (pgd_none(*pgd))
>> +		return false;
>> +
>> +	pud = pud_offset(pgd, addr);
>> +	if (pud_none(*pud))
>> +		return false;
>> +	if (pud_sect(*pud))
>> +		return true;
> 
> This wouldn't normally guarantee "present" but I don't think we ever
> have a non-present section mapping for the kernel (we do for user
> though). You may want to add a comment.

Sure.

Just in case I've totally miss-understood:
> * Because this is only called on the kernel linar map we don't need to
> * use p?d_present() to check for PROT_NONE regions, as these don't occur
> * in the linear map.


>> +
>> +	pmd = pmd_offset(pud, addr);
>> +	if (pmd_none(*pmd))
>> +		return false;
>> +	if (pmd_sect(*pmd))
>> +		return true;
>> +
>> +	pte = pte_offset_kernel(pmd, addr);
>> +	if (pte_none(*pte))
>> +		return false;
>> +
>> +	return pte_valid(*pte);
> 
> You can return pte_valid() directly without the pte_none() check since
> pte_none() implies !pte_valid().

Oops, yes.


Thanks for the comments!


James