Re: [PATCH 0/26] get_user_pages() cleanup

[KOSAKI Motohiro <kosaki.motohiro@gmail.com>]

(10/7/13 5:18 PM), Jan Kara wrote:
> On Fri 04-10-13 16:42:19, KOSAKI Motohiro wrote:
>> (10/4/13 4:31 PM), KOSAKI Motohiro wrote:
>>> (10/2/13 4:29 PM), Jan Kara wrote:
>>>> On Wed 02-10-13 09:20:09, Christoph Hellwig wrote:
>>>>> On Wed, Oct 02, 2013 at 04:27:41PM +0200, Jan Kara wrote:
>>>>>>     Hello,
>>>>>>
>>>>>>     In my quest for changing locking around page faults to make things easier for
>>>>>> filesystems I found out get_user_pages() users could use a cleanup.  The
>>>>>> knowledge about necessary locking for get_user_pages() is in tons of places in
>>>>>> drivers and quite a few of them actually get it wrong (don't have mmap_sem when
>>>>>> calling get_user_pages() or hold mmap_sem when calling copy_from_user() in the
>>>>>> surrounding code). Rather often this actually doesn't seem necessary. This
>>>>>> patch series converts lots of places to use either get_user_pages_fast()
>>>>>> or a new simple wrapper get_user_pages_unlocked() to remove the knowledge
>>>>>> of mmap_sem from the drivers. I'm still looking into converting a few remaining
>>>>>> drivers (most notably v4l2) which are more complex.
>>>>>
>>>>> Even looking over the kerneldoc comment next to it I still fail to
>>>>> understand when you'd want to use get_user_pages_fast and when not.
>>>>     AFAIU get_user_pages_fast() should be used
>>>> 1) if you don't need any special get_user_pages() arguments (like calling
>>>>      it for mm of a different process, forcing COW, or similar).
>>>> 2) you don't expect pages to be unmapped (then get_user_pages_fast() is
>>>> actually somewhat slower because it walks page tables twice).
>>>
>>> If target page point to anon or private mapping pages, get_user_pages_fast()
>>> is fork unsafe. O_DIRECT man pages describe a bit about this.
>>>
>>>
>>> see http://man7.org/linux/man-pages/man2/open.2.html
>>>
>>>>        O_DIRECT I/Os should never be run concurrently with the fork(2)
>>>>        system call, if the memory buffer is a private mapping (i.e., any
>>>>        mapping created with the mmap(2) MAP_PRIVATE flag; this includes
>>>>        memory allocated on the heap and statically allocated buffers).  Any
>>>>        such I/Os, whether submitted via an asynchronous I/O interface or
>>>>        from another thread in the process, should be completed before
>>>>        fork(2) is called.  Failure to do so can result in data corruption
>>>>        and undefined behavior in parent and child processes.  This
>>>>        restriction does not apply when the memory buffer for the O_DIRECT
>>>>        I/Os was created using shmat(2) or mmap(2) with the MAP_SHARED flag.
>>>>        Nor does this restriction apply when the memory buffer has been
>>>>        advised as MADV_DONTFORK with madvise(2), ensuring that it will not
>>>>        be available to the child after fork(2).
>>
>> IMHO, get_user_pages_fast() should be renamed to get_user_pages_quirk(). Its
>> semantics is not equal to get_user_pages(). When someone simply substitute
>> get_user_pages() to get_user_pages_fast(), they might see huge trouble.
>    I forgot about this speciality (and actually comments didn't remind me
> :(). But thinking about this some more get_user_pages_fast() seems as save
> as get_user_pages() in presence of threads sharing mm, doesn't it?

It depends.

If there is any guarantee that other threads don't touch the same page which
retrieved get_user_pages(), get_user_pages_fast() give us brilliant fast way.
Example, as far as I heard form IB guys, the userland library of the infiniband
stack uses madvise(MADV_DONTFORK), and then they don't need to care COW issue
and can choose fastest way. An another example is a futex. futex doesn't use
the contents of the pages, it uses vaddr only for looking up key. Then, it
also doesn't have COW issue.

I don't know other cases. But as far as I know, everything is case-by-case.

> Because
> while get_user_pages() are working, other thread can happilly trigger COW
> on some of the pages and thus get_user_pages() can return pages some of
> which are invisible in our mm by the time get_user_pages() returns.

If you are talking about get_user_pages() instead of get_user_pages_fast(), this
can't be happen because page-fault takes mmap_sem too.

I would say, mmap_sem has too fat responsibility really.

> So
> although in practice I agree problems of get_user_pages_fast() with fork(2)
> are more visible, in essence they are still present with clone(2) and
> get_user_pages().
>
> 								Honza
>

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