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Date:   Fri, 14 Sep 2018 15:49:05 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Jann Horn <jannh@google.com>
Cc:     Prakash Sangappa <prakash.sangappa@oracle.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Anshuman Khandual <khandual@linux.vnet.ibm.com>,
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        steven.sistare@oracle.com
Subject: Re: [RFC PATCH] Add /proc/<pid>/numa_vamaps for numa node information
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On Fri 14-09-18 14:49:10, Jann Horn wrote:
> On Fri, Sep 14, 2018 at 8:21 AM Michal Hocko <mhocko@kernel.org> wrote:
> > On Fri 14-09-18 03:33:28, Jann Horn wrote:
> > > On Wed, Sep 12, 2018 at 10:43 PM prakash.sangappa
> > > <prakash.sangappa@oracle.com> wrote:
> > > > On 05/09/2018 04:31 PM, Dave Hansen wrote:
> > > > > On 05/07/2018 06:16 PM, prakash.sangappa wrote:
> > > > >> It will be /proc/<pid>/numa_vamaps. Yes, the behavior will be
> > > > >> different with respect to seeking. Output will still be text and
> > > > >> the format will be same.
> > > > >>
> > > > >> I want to get feedback on this approach.
> > > > > I think it would be really great if you can write down a list of the
> > > > > things you actually want to accomplish.  Dare I say: you need a
> > > > > requirements list.
> > > > >
> > > > > The numa_vamaps approach continues down the path of an ever-growing list
> > > > > of highly-specialized /proc/<pid> files.  I don't think that is
> > > > > sustainable, even if it has been our trajectory for many years.
> > > > >
> > > > > Pagemap wasn't exactly a shining example of us getting new ABIs right,
> > > > > but it sounds like something along those is what we need.
> > > >
> > > > Just sent out a V2 patch.  This patch simplifies the file content. It
> > > > only provides VA range to numa node id information.
> > > >
> > > > The requirement is basically observability for performance analysis.
> > > >
> > > > - Need to be able to determine VA range to numa node id information.
> > > >    Which also gives an idea of which range has memory allocated.
> > > >
> > > > - The proc file /proc/<pid>/numa_vamaps is in text so it is easy to
> > > >    directly view.
> > > >
> > > > The V2 patch supports seeking to a particular process VA from where
> > > > the application could read the VA to  numa node id information.
> > > >
> > > > Also added the 'PTRACE_MODE_READ_REALCREDS' check when opening the
> > > > file /proc file as was indicated by Michal Hacko
> > >
> > > procfs files should use PTRACE_MODE_*_FSCREDS, not PTRACE_MODE_*_REALCREDS.
> >
> > Out of my curiosity, what is the semantic difference? At least
> > kernel_move_pages uses PTRACE_MODE_READ_REALCREDS. Is this a bug?
> 
> No, that's fine. REALCREDS basically means "look at the caller's real
> UID for the access check", while FSCREDS means "look at the caller's
> filesystem UID". The ptrace access check has historically been using
> the real UID, which is sorta weird, but normally works fine. Given
> that this is documented, I didn't see any reason to change it for most
> things that do ptrace access checks, even if the EUID would IMO be
> more appropriate. But things that capture caller credentials at points
> like open() really shouldn't look at the real UID; instead, they
> should use the filesystem UID (which in practice is basically the same
> as the EUID).
> 
> So in short, it depends on the interface you're coming through: Direct
> syscalls use REALCREDS, things that go through the VFS layer use
> FSCREDS.

Ahh, OK, I see. Thanks for the clarification! Then I agree that the proc
interface should use FSCREDS.

-- 
Michal Hocko
SUSE Labs