Re: [PATCH v2 08/10] mm/hugetlb: Make walk_hugetlb_range() safe to pmd unshare

[David Hildenbrand <david@redhat.com>]

On 08.12.22 21:47, Peter Xu wrote:
> On Thu, Dec 08, 2022 at 02:14:42PM +0100, David Hildenbrand wrote:
>> On 07.12.22 21:30, Peter Xu wrote:
>>> Since walk_hugetlb_range() walks the pgtable, it needs the vma lock
>>> to make sure the pgtable page will not be freed concurrently.
>>>
>>> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
>>> Signed-off-by: Peter Xu <peterx@redhat.com>
>>> ---
>>>    arch/s390/mm/gmap.c      |  2 ++
>>>    fs/proc/task_mmu.c       |  2 ++
>>>    include/linux/pagewalk.h | 11 ++++++++++-
>>>    mm/hmm.c                 | 15 ++++++++++++++-
>>>    mm/pagewalk.c            |  2 ++
>>>    5 files changed, 30 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
>>> index 8947451ae021..292a54c490d4 100644
>>> --- a/arch/s390/mm/gmap.c
>>> +++ b/arch/s390/mm/gmap.c
>>> @@ -2643,7 +2643,9 @@ static int __s390_enable_skey_hugetlb(pte_t *pte, unsigned long addr,
>>>    	end = start + HPAGE_SIZE - 1;
>>>    	__storage_key_init_range(start, end);
>>>    	set_bit(PG_arch_1, &page->flags);
>>> +	hugetlb_vma_unlock_read(walk->vma);
>>>    	cond_resched();
>>> +	hugetlb_vma_lock_read(walk->vma);
>>>    	return 0;
>>>    }
>>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>>> index e35a0398db63..cf3887fb2905 100644
>>> --- a/fs/proc/task_mmu.c
>>> +++ b/fs/proc/task_mmu.c
>>> @@ -1613,7 +1613,9 @@ static int pagemap_hugetlb_range(pte_t *ptep, unsigned long hmask,
>>>    			frame++;
>>>    	}
>>> +	hugetlb_vma_unlock_read(walk->vma);
>>>    	cond_resched();
>>> +	hugetlb_vma_lock_read(walk->vma);
>>
>> We already hold the mmap_lock and reschedule. Even without the
>> cond_resched() we might happily reschedule on a preemptive kernel. So I'm
>> not sure if this optimization is strictly required or even helpful in
>> practice here.
> 
> It's just low hanging fruit if we need that complexity anyway.
> 
> That's also why I didn't do that for v1 (where I missed hmm special case,
> though..), but I think since we'll need that anyway, we'd better release
> the vma lock if we can easily do so.
> 
> mmap_lock is just more special because it needs more work in the caller to
> release (e.g. vma invalidations).  Otherwise I'm happy dropping that too.
> 
>>
>> In the worst case, concurrent unsharing would have to wait.
>> For example, s390_enable_skey() is called at most once for a VM, for most
>> VMs it gets never even called.
>>
>> Or am I missing something important?
> 
> Nothing important.  I just don't see why we need to strictly follow the
> same release rule of mmap_lock here when talking about vma lock.
> 
> In short - if we can drop a lock earlier before sleep, why not?
> 
> I tend to just keep it as-is, but let me know if you have further thoughts
> or concerns.

To me this looks like a possibly unnecessary optimization, requiring 
additional code. IMHO, possibly unnecessary unlock+relock makes thatthat 
code harder to get.

For such cases, it would be good to have any evidence that it really helps.

-- 
Thanks,

David / dhildenb