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From:   Pingfan Liu <kernelfans@gmail.com>
Date:   Thu, 13 Jun 2019 18:48:14 +0800
Message-ID: <CAFgQCTsO-C=Fy6im+VQnNwvyp74tV2dZ-0Pa8QfFyFrBX8Ohvg@mail.gmail.com>
Subject: Re: [PATCHv3 1/2] mm/gup: fix omission of check on FOLL_LONGTERM in get_user_pages_fast()
To:     Ira Weiny <ira.weiny@intel.com>
Cc:     "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@linux.ibm.com>,
        "Williams, Dan J" <dan.j.williams@intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        John Hubbard <jhubbard@nvidia.com>,
        "Busch, Keith" <keith.busch@intel.com>,
        Christoph Hellwig <hch@infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Thu, Jun 13, 2019 at 7:49 AM Ira Weiny <ira.weiny@intel.com> wrote:
>
> On Wed, Jun 12, 2019 at 09:54:58PM +0800, Pingfan Liu wrote:
> > On Tue, Jun 11, 2019 at 04:29:11PM +0000, Weiny, Ira wrote:
> > > > Pingfan Liu <kernelfans@gmail.com> writes:
> > > >
> > > > > As for FOLL_LONGTERM, it is checked in the slow path
> > > > > __gup_longterm_unlocked(). But it is not checked in the fast path=
,
> > > > > which means a possible leak of CMA page to longterm pinned requir=
ement
> > > > > through this crack.
> > > >
> > > > Shouldn't we disallow FOLL_LONGTERM with get_user_pages fastpath? W=
.r.t
> > > > dax check we need vma to ensure whether a long term pin is allowed =
or not.
> > > > If FOLL_LONGTERM is specified we should fallback to slow path.
> > >
> > > Yes, the fastpath bails to the slowpath if FOLL_LONGTERM _and_ DAX.  =
But it does this while walking the page tables.  I missed the CMA case and =
Pingfan's patch fixes this.  We could check for CMA pages while walking the=
 page tables but most agreed that it was not worth it.  For DAX we already =
had checks for *_devmap() so it was easier to put the FOLL_LONGTERM checks =
there.
> > >
> > Then for CMA pages, are you suggesting something like:
>
> I'm not suggesting this.
OK, then I send out v4.
>
> Sorry I wrote this prior to seeing the numbers in your other email.  Give=
n
> the numbers it looks like performing the check whilst walking the tables =
is
> worth the extra complexity.  I was just trying to summarize the thread.  =
I
> don't think we should disallow FOLL_LONGTERM because it only affects CMA =
and
> DAX.  Other pages will be fine with FOLL_LONGTERM.  Why penalize every ca=
ll if
> we don't have to.  Also in the case of DAX the use of vma will be going
> away...[1]  Eventually...  ;-)
A good feature. Trying to catch up.

Thanks,
Pingfan