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

[Bijan Mottahedeh <bijan.mottahedeh@oracle.com>]

On 6/12/2020 8:19 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@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.
>
Are you referring to a case where one process creates a ring and sends 
the ring fd to another process?