Re: [PATCH 3/5] mm/mremap: use pmd_addr_end to calculate next in move_page_tables()

[Dmitry Osipenko]

29.01.2020 12:47, Russell King - ARM Linux admin ÐÐÑÐÑ:
> On Sun, Jan 26, 2020 at 05:47:57PM +0300, Dmitry Osipenko wrote:
>> 18.01.2020 02:22, Wei Yang ÐÐÑÐÑ:
>>> Use the general helper instead of do it by hand.
>>>
>>> Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
>>> ---
>>>  mm/mremap.c | 7 ++-----
>>>  1 file changed, 2 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/mm/mremap.c b/mm/mremap.c
>>> index c2af8ba4ba43..a258914f3ee1 100644
>>> --- a/mm/mremap.c
>>> +++ b/mm/mremap.c
>>> @@ -253,11 +253,8 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
>>>  
>>>  	for (; old_addr < old_end; old_addr += extent, new_addr += extent) {
>>>  		cond_resched();
>>> -		next = (old_addr + PMD_SIZE) & PMD_MASK;
>>> -		/* even if next overflowed, extent below will be ok */
>>> +		next = pmd_addr_end(old_addr, old_end);
>>>  		extent = next - old_addr;
>>> -		if (extent > old_end - old_addr)
>>> -			extent = old_end - old_addr;
>>>  		old_pmd = get_old_pmd(vma->vm_mm, old_addr);
>>>  		if (!old_pmd)
>>>  			continue;
>>> @@ -301,7 +298,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
>>>  
>>>  		if (pte_alloc(new_vma->vm_mm, new_pmd))
>>>  			break;
>>> -		next = (new_addr + PMD_SIZE) & PMD_MASK;
>>> +		next = pmd_addr_end(new_addr, new_addr + len);
>>>  		if (extent > next - new_addr)
>>>  			extent = next - new_addr;
>>>  		move_ptes(vma, old_pmd, old_addr, old_addr + extent, new_vma,
>>>
>>
>> Hello Wei,
>>
>> Starting with next-20200122, I'm seeing the following in KMSG on NVIDIA
>> Tegra (ARM32):
>>
>>   BUG: Bad rss-counter state mm:(ptrval) type:MM_ANONPAGES val:190
>>
>> and eventually kernel hangs.
>>
>> Git's bisection points to this patch and reverting it helps. Please fix,
>> thanks in advance.
> 
> The above is definitely wrong - pXX_addr_end() are designed to be used
> with an address index within the pXX table table and the address index
> of either the last entry in the same pXX table or the beginning of the
> _next_ pXX table.  Arbitary end address indicies are not allowed.
> 
> When page tables are "rolled up" when levels don't exist, it is common
> practice for these macros to just return their end address index.
> Hence, if they are used with arbitary end address indicies, then the
> iteration will fail.
> 
> The only way to do this is:
> 
> 	next = pmd_addr_end(old_addr,
> 			pud_addr_end(old_addr,
> 				p4d_addr_end(old_addr,
> 					pgd_addr_end(old_addr, old_end))));
> 
> which gives pmd_addr_end() (and each of the intermediate pXX_addr_end())
> the correct end argument.  However, that's a more complex and verbose,
> and likely less efficient than the current code.
> 
> I'd suggest that there's nothing to "fix" in the v5.5 code wrt this,
> and trying to "clean it up" will just result in less efficient or
> broken code.
> 

Hello Russell,

Thank you very much for the extra clarification!