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

[Pavel Begunkov <asml.silence@gmail.com>]

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).

-- 
Pavel Begunkov