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[91.12.96.79]) by smtp.gmail.com with ESMTPSA id c5sm19156137wrd.13.2021.11.10.00.30.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 10 Nov 2021 00:30:52 -0800 (PST) Message-ID: <793685d2-be3f-9a74-c9a3-65c486e0ef1f@redhat.com> Date: Wed, 10 Nov 2021 09:30:50 +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> 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-kernel@vger.kernel.org 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." > >> >> 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