Re: [PATCH] mm: add account_locked_vm utility function

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 21/05/2019 01:30, Daniel Jordan wrote:
> On Mon, May 20, 2019 at 04:19:34PM +1000, Alexey Kardashevskiy wrote:
>> On 04/05/2019 06:16, Daniel Jordan wrote:
>>> locked_vm accounting is done roughly the same way in five places, so
>>> unify them in a helper.  Standardize the debug prints, which vary
>>> slightly.
>>
>> And I rather liked that prints were different and tell precisely which
>> one of three each printk is.
> 
> I'm not following.  One of three...callsites?  But there were five callsites.


Well, 3 of them are mine, I was referring to them :)


> Anyway, I added a _RET_IP_ to the debug print so you can differentiate.


I did not know that existed, cool!


> 
>> I commented below but in general this seems working.
>>
>> Tested-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> 
> Thanks!  And for the review as well.
> 
>>> diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
>>> index 6b64e45a5269..d39a1b830d82 100644
>>> --- a/drivers/vfio/vfio_iommu_spapr_tce.c
>>> +++ b/drivers/vfio/vfio_iommu_spapr_tce.c
>>> @@ -34,49 +35,13 @@
>>>  static void tce_iommu_detach_group(void *iommu_data,
>>>  		struct iommu_group *iommu_group);
>>>  
>>> -static long try_increment_locked_vm(struct mm_struct *mm, long npages)
>>> +static int tce_account_locked_vm(struct mm_struct *mm, unsigned long npages,
>>> +				 bool inc)
>>>  {
>>> -	long ret = 0, locked, lock_limit;
>>> -
>>>  	if (WARN_ON_ONCE(!mm))
>>>  		return -EPERM;
>>
>>
>> If this WARN_ON is the only reason for having tce_account_locked_vm()
>> instead of calling account_locked_vm() directly, you can then ditch the
>> check as I have never ever seen this triggered.
> 
> Great, will do.
> 
>>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>>> index d0f731c9920a..15ac76171ccd 100644
>>> --- a/drivers/vfio/vfio_iommu_type1.c
>>> +++ b/drivers/vfio/vfio_iommu_type1.c
>>> @@ -273,25 +273,14 @@ static int vfio_lock_acct(struct vfio_dma *dma, long npage, bool async)
>>>  		return -ESRCH; /* process exited */
>>>  
>>>  	ret = down_write_killable(&mm->mmap_sem);
>>> -	if (!ret) {
>>> -		if (npage > 0) {
>>> -			if (!dma->lock_cap) {
>>> -				unsigned long limit;
>>> -
>>> -				limit = task_rlimit(dma->task,
>>> -						RLIMIT_MEMLOCK) >> PAGE_SHIFT;
>>> -
>>> -				if (mm->locked_vm + npage > limit)
>>> -					ret = -ENOMEM;
>>> -			}
>>> -		}
>>> +	if (ret)
>>> +		goto out;
>>
>>
>> A single "goto" to jump just 3 lines below seems unnecessary.
> 
> No strong preference here, I'll take out the goto.
> 
>>> +int __account_locked_vm(struct mm_struct *mm, unsigned long pages, bool inc,
>>> +			struct task_struct *task, bool bypass_rlim)
>>> +{
>>> +	unsigned long locked_vm, limit;
>>> +	int ret = 0;
>>> +
>>> +	locked_vm = mm->locked_vm;
>>> +	if (inc) {
>>> +		if (!bypass_rlim) {
>>> +			limit = task_rlimit(task, RLIMIT_MEMLOCK) >> PAGE_SHIFT;
>>> +			if (locked_vm + pages > limit) {
>>> +				ret = -ENOMEM;
>>> +				goto out;
>>> +			}
>>> +		}
>>
>> Nit:
>>
>> if (!ret)
>>
>> and then you don't need "goto out".
> 
> Ok, sure.
> 
>>> +		mm->locked_vm = locked_vm + pages;
>>> +	} else {
>>> +		WARN_ON_ONCE(pages > locked_vm);
>>> +		mm->locked_vm = locked_vm - pages;
>>
>>
>> Can go negative here. Not a huge deal but inaccurate imo.
> 
> I hear you, but setting a negative value to zero, as we had done previously,
> doesn't make much sense to me.


Ok then. I have not seen these WARN_ON for a very long time anyway.


-- 
Alexey