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

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Date:   Fri, 18 Sep 2020 07:51:59 +0530
From:   Souptick Joarder <jrdr.linux@...il.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Ira Weiny <ira.weiny@...el.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        mporter@...nel.crashing.org, alex.bou9@...il.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        gustavoars@...nel.org, madhuparnabhowmik10@...il.com,
        linux-kernel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>
Subject: Re: [linux-next PATCH] rapidio: Fix error handling path

On Thu, Sep 17, 2020 at 11:17 PM John Hubbard <jhubbard@...dia.com> wrote:
>
> 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

There was some discussion on removing return value 0 from one of the
gup variants [1].
I think it might be partially relevant to the current discussion.

[1] https://patchwork.kernel.org/patch/11529795/

>
> >
> > 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