Messages in this thread | | | Subject | Re: [RFC PATCH] Add /proc/<pid>/numa_vamaps for numa node information | From | Prakash Sangappa <> | Date | Fri, 14 Sep 2018 11:07:53 -0700 |
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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?
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