Re: [PATCH v4] mm: Add PM_HUGE_THP_MAPPING to /proc/pid/pagemap

Mina Almasry
2021-11-10T17:50:53Z

On Wed, Nov 10, 2021 at 12:57 AM Peter Xu <peterx@redhat.com> wrote:
>
>
> This also reminded me that we've got issue with smaps being too slow, and in
> many cases we're only interested in a small portion of the whole memory. This
> made me wonder how about a new smaps interface taking memory range as input.
>

Does a patch like I'm providing here address the perf issues you're seeing?

> Thanks,
>
> --
> Peter Xu
>

On Wed, Nov 10, 2021 at 2:24 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 10.11.21 09:57, Peter Xu wrote:
> > On Wed, Nov 10, 2021 at 09:30:50AM +0100, David Hildenbrand wrote:
> >> On 10.11.21 09:27, Peter Xu wrote:
> >>> On Wed, Nov 10, 2021 at 09:14:42AM +0100, David Hildenbrand wrote:
> >>>> On 10.11.21 08:03, Peter Xu wrote:
> >>>>> Hi, Mina,
> >>>>>
> >>>>> Sorry to comment late.
> >>>>>
> >>>>> On Sun, Nov 07, 2021 at 03:57:54PM -0800, Mina Almasry wrote:
> >>>>>> diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
> >>>>>> index fdc19fbc10839..8a0f0064ff336 100644
> >>>>>> --- a/Documentation/admin-guide/mm/pagemap.rst
> >>>>>> +++ b/Documentation/admin-guide/mm/pagemap.rst
> >>>>>> @@ -23,7 +23,8 @@ There are four components to pagemap:
> >>>>>> * Bit 56 page exclusively mapped (since 4.2)
> >>>>>> * Bit 57 pte is uffd-wp write-protected (since 5.13) (see
> >>>>>> :ref:`Documentation/admin-guide/mm/userfaultfd.rst <userfaultfd>`)
> >>>>>> - * Bits 57-60 zero
> >>>>>> + * Bit 58 page is a huge (PMD size) THP mapping
> >>>>>> + * Bits 59-60 zero
> >>>>>> * Bit 61 page is file-page or shared-anon (since 3.5)
> >>>>>> * Bit 62 page swapped
> >>>>>> * Bit 63 page present
> >>>>>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> >>>>>> index ad667dbc96f5c..6f1403f83b310 100644
> >>>>>> --- a/fs/proc/task_mmu.c
> >>>>>> +++ b/fs/proc/task_mmu.c
> >>>>>> @@ -1302,6 +1302,7 @@ struct pagemapread {
> >>>>>> #define PM_SOFT_DIRTY BIT_ULL(55)
> >>>>>> #define PM_MMAP_EXCLUSIVE BIT_ULL(56)
> >>>>>> #define PM_UFFD_WP BIT_ULL(57)
> >>>>>> +#define PM_HUGE_THP_MAPPING BIT_ULL(58)
> >>>>>
> >>>>> The ending "_MAPPING" seems redundant to me, how about just call it "PM_THP" or
> >>>>> "PM_HUGE" (as THP also means HUGE already)?
> >>>>>
> >>>>> IMHO the core problem is about permission controls, and it seems to me we're
> >>>>> actually trying to workaround it by duplicating some information we have.. so
> >>>>> it's kind of a pity. Totally not against this patch, but imho it'll be nicer
> >>>>> if it's the permission part that to be enhanced, rather than a new but slightly
> >>>>> duplicated interface.
> >>>>
> >>>> It's not a permission problem AFAIKS: even with permissions "changed",
> >>>> any attempt to use /proc/kpageflags is just racy. Let's not go down that
> >>>> path, it's really the wrong mechanism to export to random userspace.
> >>>
> >>> I agree it's racy, but IMHO that's fine. These are hints for userspace to make
> >>> decisions, they cannot be always right. Even if we fetch atomically and seeing
> >>> that this pte is swapped out, it can be quickly accessed at the same time and
> >>> it'll be in-memory again. Only if we can freeze the whole pgtable but we
> >>> can't, so they can only be used as hints.
> >>
> >> Sorry, I don't think /proc/kpageflags (or exporting the PFNs to random
> >> users via /proc/self/pagemap) is the way to go.
> >>
> >> "Since Linux 4.0 only users with the CAP_SYS_ADMIN capability can get
> >> PFNs. In 4.0 and 4.1 opens by unprivileged fail with -EPERM. Starting
> >> from 4.2 the PFN field is zeroed if the user does not have
> >> CAP_SYS_ADMIN. Reason: information about PFNs helps in exploiting
> >> Rowhammer vulnerability."
> >
> > IMHO these are two problems that you mentioned. That's also what I was
> > wondering about: could the app be granted with CAP_SYS_ADMIN then?
> >
> > I am not sure whether that'll work well with /proc/kpage* though, as it's by
> > default 0400. So perhaps we need to manual adjust the file permission too to
> > make sure the app can both access PFNs (with SYS_ADMIN) and the flags. Totally
> > no expert on the permissions..
>
> Me too :)
>
> IIRC changing permissions that was not an option -- which is why the
> first approach suggested a new /proc/self/pageflags. But I guess Mina
> can remind us (and eventually document all that in the patch description
> :) ).
>

Sorry, yes I should update the commit message with this info. The
issues with smaps are:
1. Performance: I've pinged our network service folks to obtain a
rough perf comparison but I haven't been able to get one. I can try to
get a performance measurement myself but Peter seems to be also seeing
this.
2. smaps output is human readable and a bit convoluted for userspace to parse.

>
> --
> Thanks,
>
> David / dhildenb
>