Re: [RFC PATCH] Add /proc/<pid>/numa_vamaps for numa node information

[Prakash Sangappa <prakash.sangappa@oracle.com>]

On 9/14/18 5:49 AM, 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.

So in this case can the REALCREDS check be done in the read() system call
when reading the /proc file instead of the open call?