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).
Honza
--
Jan Kara <[email protected]>
SUSE Labs, CR
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