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

[Jens Axboe <axboe@kernel.dk>]

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@oracle.com>
>>
>> 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.

-- 
Jens Axboe