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Message-ID: <793685d2-be3f-9a74-c9a3-65c486e0ef1f@redhat.com>
Date:   Wed, 10 Nov 2021 09:30:50 +0100
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Subject: Re: [PATCH v4] mm: Add PM_HUGE_THP_MAPPING to /proc/pid/pagemap
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To:     Peter Xu <peterx@redhat.com>
Cc:     Mina Almasry <almasrymina@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        "Paul E . McKenney" <paulmckrcu@fb.com>,
        Yu Zhao <yuzhao@google.com>, Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ivan Teterevkov <ivan.teterevkov@nutanix.com>,
        Florian Schmidt <florian.schmidt@nutanix.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org
References: <20211107235754.1395488-1-almasrymina@google.com>
 <YYtuqsnOSxA44AUX@t490s> <c5ed86d0-8af6-f54f-e352-8871395ad62e@redhat.com>
 <YYuCaNXikls/9JhS@t490s>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <YYuCaNXikls/9JhS@t490s>
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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