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[91.12.96.79]) by smtp.gmail.com with ESMTPSA id p13sm5552185wmi.0.2021.11.10.02.24.11 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 10 Nov 2021 02:24:11 -0800 (PST) Message-ID: <8032a24c-3800-16e5-41b7-5565e74d3863@redhat.com> Date: Wed, 10 Nov 2021 11:24:10 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0 Subject: Re: [PATCH v4] mm: Add PM_HUGE_THP_MAPPING to /proc/pid/pagemap Content-Language: en-US To: Peter Xu Cc: Mina Almasry , Matthew Wilcox , "Paul E . McKenney" , Yu Zhao , Jonathan Corbet , Andrew Morton , Ivan Teterevkov , Florian Schmidt , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org References: <20211107235754.1395488-1-almasrymina@google.com> <793685d2-be3f-9a74-c9a3-65c486e0ef1f@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org 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 `) >>>>>> - * 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 :) ). -- Thanks, David / dhildenb