(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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