Re: [RFC 1/1] io_uring: preserve work->mm since actual work processing may need it

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

On 4/10/2020 10:51 AM, Pavel Begunkov wrote:
> On 10/04/2020 19:54, Bijan Mottahedeh wrote:
>>> As I see, this down_read() from the trace is
>>> down_read(&current->mm->mmap_sem), where current->mm is set by use_mm()
>>> just several lines above your change. So, what do you mean by passing? I
>>> don't see do_madvise() __explicitly__ accepting mm as an argument.
>> I think the sequence is:
>>
>> io_madvise()
>> -> do_madvise(NULL, req->work.mm, ma->addr, ma->len, ma->advice)
>>                      ^^^^^^^^^^^^
>>     -> down_read(&mm->mmap_sem)
>>
>> I added an assert in do_madvise() for a NULL mm value and hit it running the test.
>>
>>> What tree do you use? Extra patches on top?
>> I'm using next-20200409 with no patches.
> I see, it came from 676a179 ("mm: pass task and mm to do_madvise"), which isn't
> in Jen's tree.
>
> I don't think your patch will do, because it changes mm refcounting with extra
> mmdrop() in io_req_work_drop_env(). That's assuming it worked well before.
>
> Better fix then is to make it ```do_madvise(NULL, current->mm, ...)```
> as it actually was at some point in the mentioned patch (v5).
>
Ok. Jens had suggested to use req->work.mm in the patch comments so 
let's just get him to confirm:

"I think we want to use req->work.mm here - it'll be the same as
current->mm at this point, but it makes it clear that we're using a
grabbed mm."