On Fri 14-09-18 14:49:10, Jann Horn wrote: > On Fri, Sep 14, 2018 at 8:21 AM Michal Hocko <mhocko@xxxxxxxxxx> 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@xxxxxxxxxx> 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
