Re: [RFC 2/2] io_uring: report pinned memory usage

[Jens Axboe <axboe@xxxxxxxxx>]

On 6/12/20 9:19 AM, Jens Axboe wrote:
> On 6/12/20 9:16 AM, Jens Axboe wrote:
>> On 6/11/20 8:23 PM, Bijan Mottahedeh wrote:
>>> Long term, it makes sense to separate reporting and enforcing of pinned
>>> memory usage.
>>>
>>> Signed-off-by: Bijan Mottahedeh <bijan.mottahedeh@xxxxxxxxxx>
>>>
>>> It is useful to view
>>> ---
>>>  fs/io_uring.c | 4 ++++
>>>  1 file changed, 4 insertions(+)
>>>
>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>> index 4248726..cf3acaa 100644
>>> --- a/fs/io_uring.c
>>> +++ b/fs/io_uring.c
>>> @@ -7080,6 +7080,8 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx,
>>>  static void io_unaccount_mem(struct user_struct *user, unsigned long nr_pages)
>>>  {
>>>  	atomic_long_sub(nr_pages, &user->locked_vm);
>>> +	if (current->mm)
>>> +		atomic_long_sub(nr_pages, &current->mm->pinned_vm);
>>>  }
>>>  
>>>  static int io_account_mem(struct user_struct *user, unsigned long nr_pages)
>>> @@ -7096,6 +7098,8 @@ static int io_account_mem(struct user_struct *user, unsigned long nr_pages)
>>>  			return -ENOMEM;
>>>  	} while (atomic_long_cmpxchg(&user->locked_vm, cur_pages,
>>>  					new_pages) != cur_pages);
>>> +	if (current->mm)
>>> +		atomic_long_add(nr_pages, &current->mm->pinned_vm);
>>>  
>>>  	return 0;
>>>  }
>>
>> current->mm should always be valid for these, so I think you can skip the
>> checking of that and just make it unconditional.
> 
> Two other issues with this:
> 
> - It's an atomic64, so seems more appropriate to use the atomic64 helpers
>   for this one.
> - The unaccount could potentially be a different mm, if the ring is shared
>   and one task sets it up while another tears it down. So we'd need something
>   to ensure consistency here.

IOW, this should just use ctx->sqo_mm, as that's grabbed and valid. Just need
to ensure it's done correct in terms of ordering for setup.

-- 
Jens Axboe