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

[Jan Kara <jack@suse.cz>]

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? 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. 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
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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