Re: [PATCH v3 2/7] mm: Add a walk_page_mapping() function to the pagewalk code

["Thomas Hellström (VMware)" <thomas_os@shipmail.org>]

On 10/4/19 2:37 PM, Kirill A. Shutemov wrote:
> On Thu, Oct 03, 2019 at 01:32:45PM +0200, Thomas Hellström (VMware) wrote:
>>>> + *   If @mapping allows faulting of huge pmds and puds, it is desirable
>>>> + *   that its huge_fault() handler blocks while this function is running on
>>>> + *   @mapping. Otherwise a race may occur where the huge entry is split when
>>>> + *   it was intended to be handled in a huge entry callback. This requires an
>>>> + *   external lock, for example that @mapping->i_mmap_rwsem is held in
>>>> + *   write mode in the huge_fault() handlers.
>>> Em. No. We have ptl for this. It's the only lock required (plus mmap_sem
>>> on read) to split PMD entry into PTE table. And it can happen not only
>>> from fault path.
>>>
>>> If you care about splitting compound page under you, take a pin or lock a
>>> page. It will block split_huge_page().
>>>
>>> Suggestion to block fault path is not viable (and it will not happen
>>> magically just because of this comment).
>>>
>> I was specifically thinking of this:
>>
>> https://elixir.bootlin.com/linux/latest/source/mm/pagewalk.c#L103
>>
>> If a huge pud is concurrently faulted in here, it will immediatly get split
>> without getting processed in pud_entry(). An external lock would protect
>> against that, but that's perhaps a bug in the pagewalk code?  For pmds the
>> situation is not the same since when pte_entry is used, all pmds will
>> unconditionally get split.
> I *think* it should be fixed with something like this (there's no
> pud_trans_unstable() yet):
>
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index d48c2a986ea3..221a3b945f42 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -102,10 +102,11 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
>   					break;
>   				continue;
>   			}
> +		} else {
> +			split_huge_pud(walk->vma, pud, addr);
>   		}
>   
> -		split_huge_pud(walk->vma, pud, addr);
> -		if (pud_none(*pud))
> +		if (pud_none(*pud) || pud_trans_unstable(*pud))
>   			goto again;
>   
>   		if (ops->pmd_entry || ops->pte_entry)

Yes, this seems better. I was looking at implementing a 
pud_trans_unstable() as a basis of fixing problems like this, but when I 
looked at pmd_trans_unstable I got a bit confused:

Why are devmap huge pmds considered stable? I mean, couldn't anybody 
just run madvise() to clear those just like transhuge pmds?

>
> Or better yet converted to what we do on pmd level.
>
> Honestly, all the code around PUD THP missing a lot of ground work.
> Rushing it upstream for DAX was not a right move.
>
>> There's a similar more scary race in
>>
>> https://elixir.bootlin.com/linux/latest/source/mm/memory.c#L3931
>>
>> It looks like if a concurrent thread faults in a huge pud just after the
>> test for pud_none in that pmd_alloc, things might go pretty bad.
> Hm? It will fail the next pmd_none() check under ptl. Do you have a
> particular racing scenarion?
>
Yes, I misinterpreted the code somewhat, but here's the scenario that 
looks racy:

Thread 1		Thread 2
huge_fault(pud)					- Fell back, for example because of write fault on dirty-tracking.
			huge_fault(pud)         - Taken, read fault.
pmd_alloc()                                     - Will fail pmd_none check and return a pmd_offset()
                                                   into thread 2's THP.

Thanks,

Thomas