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

David Hildenbrand
2021-11-10T08:31:00Z

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."

>
>>
>> We do have an interface to access this information from userspace
>> already: /proc/self/smaps IIRC. Mina commented that they are seeing
>> performance issues with that approach.
>>
>> It would be valuable to add these details to the patch description,
>> including a performance difference when using both interfaces we have
>> available. As the patch description stands, there is no explanation
>> "why" we want this change.
>
> I didn't notice Mina mention about performance issues with kpageflags, if so
> then I agree this solution helps.
The performance issue seems to be with /proc/self/smaps.

--
Thanks,

David / dhildenb