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From:   Souptick Joarder <jrdr.linux@gmail.com>
Date:   Wed, 6 May 2020 17:51:39 +0530
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Subject: Re: [RFC] mm/gup.c: Updated return value of {get|pin}_user_pages_fast()
To:     Jan Kara <jack@suse.cz>
Cc:     John Hubbard <jhubbard@nvidia.com>,
        Tony Luck <tony.luck@intel.com>, fenghua.yu@intel.com,
        Rob Springer <rspringer@google.com>,
        Todd Poynor <toddpoynor@google.com>, benchan@chromium.org,
        Greg KH <gregkh@linuxfoundation.org>,
        Jens Wiklander <jens.wiklander@linaro.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        santosh.shilimkar@oracle.com,
        "David S. Miller" <davem@davemloft.net>, kuba@kernel.org,
        Ira Weiny <ira.weiny@intel.com>,
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        rds-devel@oss.oracle.com
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On Wed, May 6, 2020 at 3:36 PM Jan Kara <jack@suse.cz> wrote:
>
> On Wed 06-05-20 02:06:56, Souptick Joarder wrote:
> > On Wed, May 6, 2020 at 1:08 AM John Hubbard <jhubbard@nvidia.com> wrote:
> > >
> > > On 2020-05-05 12:14, Souptick Joarder wrote:
> > > > Currently {get|pin}_user_pages_fast() have 3 return value 0, -errno
> > > > and no of pinned pages. The only case where these two functions will
> > > > return 0, is for nr_pages <= 0, which doesn't find a valid use case.
> > > > But if at all any, then a -ERRNO will be returned instead of 0, which
> > > > means {get|pin}_user_pages_fast() will have 2 return values -errno &
> > > > no of pinned pages.
> > > >
> > > > Update all the callers which deals with return value 0 accordingly.
> > >
> > > Hmmm, seems a little shaky. In order to do this safely, I'd recommend
> > > first changing gup_fast/pup_fast so so that they return -EINVAL if
> > > the caller specified nr_pages==0, and of course auditing all callers,
> > > to ensure that this won't cause problems.
> >
> > While auditing it was figured out, there are 5 callers which cares for
> > return value
> > 0 of gup_fast/pup_fast. What problem it might cause if we change
> > gup_fast/pup_fast
> > to return -EINVAL and update all the callers in a single commit ?
>
> Well, first I'd ask a different question: Why do you want to change the
> current behavior? It's not like the current behavior is confusing.  Callers
> that pass >0 pages can happily rely on the simple behavior of < 0 return on
> error or > 0 return if we mapped some pages. Callers that can possibly ask
> to map 0 pages can get 0 pages back - kind of expected - and I don't see
> any benefit in trying to rewrite these callers to handle -EINVAL instead...

Callers with a request to map 0 pages doesn't have a valid use case. But if any
caller end up doing it mistakenly, -errno should be returned to caller
rather than 0
which will indicate more precisely that map 0 pages is not a valid
request from caller.
With these, 3rd return value 0, is no more needed.

That was the thought behind this proposal.