linux-kernel - Re: [PATCH v2 0/3] mm/get_user_pages

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Date:   Sun, 8 Apr 2018 06:08:26 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Huang Ying <ying.huang@...el.com>,
        Jonathan Corbet <corbet@....net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Thorsten Leemhuis <regressions@...mhuis.info>
Subject: Re: [PATCH v2 0/3] mm/get_user_pages_fast fixes, cleanups

On Fri, Apr 06, 2018 at 01:08:02PM -0700, Andrew Morton wrote:
> On Fri, 6 Apr 2018 00:03:48 +0300 "Michael S. Tsirkin" <mst@...hat.com> wrote:
> 
> > Turns out get_user_pages_fast and __get_user_pages_fast return different
> > values on error when given a single page: __get_user_pages_fast returns
> > 0.  get_user_pages_fast returns either 0 or an error.
> > 
> > Callers of get_user_pages_fast expect an error so fix it up to return an
> > error consistently.
> > 
> > Stress the difference between get_user_pages_fast and __get_user_pages_fast
> > to make sure callers aren't confused.
> > 
> 
> A term which is missing from all these changelogs is "vhost" :(

vhost has a BUG_ON for unexpected handling so it catches the bug, but
it's not the only site affected.

> This patchset fixes a user-affecting bug, does it not?  If so, please
> fully describe that bug so that we can decide which kernel version(s)
> need the patchset.

OK, I'll try to write up something.

> And yes, this return value asymmetry is sad.  Did you scope out what
> would be needed to fix up the callers so we can avoid this?

Yes - there is a very small number of callers for __get_user_pages_fast
so it's easy to teach them all to treat any value <=0 as 0.
There seems to be some opposition to changing the API,
I'd like to look into this after the bugfix patchset is merged.

-- 
MST