Re: [linux-next PATCH] rapidio: Fix error handling path

[John Hubbard]

On 9/17/20 10:34 AM, Ira Weiny wrote:
> On Thu, Sep 17, 2020 at 03:39:51PM +0300, Dan Carpenter wrote:
> > On Wed, Sep 16, 2020 at 01:02:32PM +0300, Dan Carpenter wrote:
> > > On Wed, Sep 16, 2020 at 09:12:17AM +0530, Souptick Joarder wrote:
> > > > There is an error when pin_user_pages_fast() returns -ERRNO and
> > > > inside error handling path driver end up calling unpin_user_pages()
> > > > with -ERRNO which is not correct.
> > > >
> > > > This patch will fix the problem.
> > >
> > > There are a few ways we could prevent bug in the future.
> > >
> > > 1) This could have been caught with existing static analysis tools
> > >     which warn about when a value is set but not used.
> > >
> > > 2) I've created a Smatch check which warngs about:
> > >
> > > 	drivers/rapidio/devices/rio_mport_cdev.c:955 rio_dma_transfer() warn: unpinning negative pages 'nr_pages'
> > >
> > >     I'll test it out tonight and see how well it works.  I don't
> > >     immediately see any other bugs allthough Smatch doesn't like the code
> > >     in siw_umem_release().  It uses "min_t(int" which suggests that
> > >     negative pages are okay.
> > >
> > > 	   int to_free = min_t(int, PAGES_PER_CHUNK, num_pages);
> >
> > I only found one bug but I'm going to add unpin_user_pages_dirty_lock()
> > to the mix a retest.  There were a few other false positives.  In
> > reviewing the code, I noticed that orangefs_bufmap_map() is also buggy.
> >
> > I sort of feel like returning partial successes is not working.  We
> > could easily make a wrapper which either pins everything or it returns
> > an error code.

Yes we could. And I have the same feeling about this API. It's generated a
remarkable amount of bug fixes, several of which ended up being partial or
wrong in themselves. And mostly this is due to the complicated tristate
return code: instead of 0 or -ERRNO, it also can return "N pages that is
less than what you requested", and there are no standard helpers in the kernel
to make that easier to deal with.

> I guess the question is are there drivers which will keep working (or limp
> along?) on partial pins?  A quick search of a driver I thought did this does
> not apparently any more...  So it sounds good to me from 30,000 feet!  :-D

It sounds good to me too--and from just a *few hundred feet* (having touched most
of the call sites at some point)! haha :)

I think the wrapper should be short-term, though, just until all the callers
are converted to the simpler API. Then change the core gup/pup calls to the simpler
API. There are more than enough gup/pup API entry points as it is, that's for sure.


thanks,
--
John Hubbard
NVIDIA