From: David Herrmann <dh.herrmann@gmail.com>
To: Mike Kravetz <mike.kravetz@oracle.com>
Cc: "Marc-André Lureau" <marcandre.lureau@redhat.com>,
linux-mm <linux-mm@kvack.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
aarcange@redhat.com, "Hugh Dickins" <hughd@google.com>,
nyc@holomorphy.com
Subject: Re: [PATCH 4/6] hugetlbfs: implement memfd sealing
Date: Sun, 5 Nov 2017 13:07:15 +0100 [thread overview]
Message-ID: <CANq1E4Q7FVpW8a_M2zzrmu+ZryLskVdpSffcPXEax2rPNubhuw@mail.gmail.com> (raw)
In-Reply-To: <c6c1c10f-a572-bdda-fe5d-5c28ce1c7a11@oracle.com>
Hi
On Sat, Nov 4, 2017 at 12:31 AM, Mike Kravetz <mike.kravetz@oracle.com> wrote:
> On 11/03/2017 10:56 AM, Mike Kravetz wrote:
>> On 11/03/2017 10:41 AM, David Herrmann wrote:
>>> Hi
>>>
>>> On Fri, Nov 3, 2017 at 6:12 PM, Mike Kravetz <mike.kravetz@oracle.com> wrote:
>>>> On 11/03/2017 10:03 AM, David Herrmann wrote:
>>>>> Hi
>>>>>
>>>>> On Tue, Oct 31, 2017 at 7:40 PM, Marc-André Lureau
>>>>> <marcandre.lureau@redhat.com> wrote:
>>>>>> Implements memfd sealing, similar to shmem:
>>>>>> - WRITE: deny fallocate(PUNCH_HOLE). mmap() write is denied in
>>>>>> memfd_add_seals(). write() doesn't exist for hugetlbfs.
>>>>>> - SHRINK: added similar check as shmem_setattr()
>>>>>> - GROW: added similar check as shmem_setattr() & shmem_fallocate()
>>>>>>
>>>>>> Except write() operation that doesn't exist with hugetlbfs, that
>>>>>> should make sealing as close as it can be to shmem support.
>>>>>
>>>>> SEAL, SHRINK, and GROW look fine to me.
>>>>>
>>>>> Regarding WRITE
>>>>
>>>> The commit message may not be clear. However, hugetlbfs does not support
>>>> the write system call (or aio). The only way to modify contents of a
>>>> hugetlbfs file is via mmap or hole punch/truncate. So, we do not really
>>>> need to worry about those special (a)io cases for hugetlbfs.
>>>
>>> This is not about the write(2) syscall. Please consider this scenario
>>> about shmem:
>>>
>>> You create a memfd via memfd_create() and map it writable. You now
>>> call another kernel syscall that takes as input _any mapped page
>>> range_. You pass your mapped memfd-addresses to it. Those syscalls
>>> tend to use get_user_pages() to pin arbitrary user-mapped pages, as
>>> such this also affects shmem. In this case, those pages might stay
>>> mapped even if you munmap() your memfd!
>>>
>>> One example of this is using AIO-read() on any other file that
>>> supports it, passing your mapped memfd as buffer to _read into_. The
>>> operations supported on the memfd are irrelevant here.
>>> The selftests contain a FUSE-based test for this, since FUSE allows
>>> user-space to GUP pages for an arbitrary amount of time.
>>>
>>> The original fix for this is:
>>>
>>> commit 05f65b5c70909ef686f865f0a85406d74d75f70f
>>> Author: David Herrmann <dh.herrmann@gmail.com>
>>> Date: Fri Aug 8 14:25:36 2014 -0700
>>>
>>> shm: wait for pins to be released when sealing
>>>
>>> Please have a look at this. Your patches use shmem_add_seals() almost
>>> unchanged, and as such you call into shmem_wait_for_pins() on
>>> hugetlbfs. I would really like to see an explicit ACK that this works
>>> on hugetlbfs.
>>
>> Thanks for the explanation. I missed that in your first reply. I'll
>> look into this for hugetlbfs.
>
> I reviewed the routines in the above commit and did not see anything that
> would prevent them from working properly with hugetlbfs. I modified the
> fuse test to use hugetlbfs based mapping. I also instrumented the above
> routines and verified that tags were set/checked/cleared as designed for
> hugetlb pages. So, that is an ACK on working with hugetlbfs.
>
> This does bring up the point that the fuse seals test should also be
> modified to work with hugetlbfs as part of this series.
Perfect! Looks all good to me then!
Thanks
David
next prev parent reply other threads:[~2017-11-05 12:07 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-10-31 18:40 [PATCH 0/6] memfd: add sealing to hugetlb-backed memory Marc-André Lureau
2017-10-31 18:40 ` [PATCH 1/6] shmem: unexport shmem_add_seals()/shmem_get_seals() Marc-André Lureau
2017-11-01 22:50 ` Mike Kravetz
2017-10-31 18:40 ` [PATCH 2/6] shmem: rename functions that are memfd-related Marc-André Lureau
2017-11-01 23:01 ` Mike Kravetz
2017-11-03 16:02 ` Marc-André Lureau
2017-11-03 16:22 ` Mike Kravetz
2017-11-03 16:36 ` Marc-André Lureau
2017-11-03 18:07 ` Mike Kravetz
2017-10-31 18:40 ` [PATCH 3/6] hugetlb: expose hugetlbfs_inode_info in header Marc-André Lureau
2017-11-01 23:20 ` Mike Kravetz
2017-11-03 16:14 ` Marc-André Lureau
2017-11-03 16:23 ` Mike Kravetz
2017-10-31 18:40 ` [PATCH 4/6] hugetlbfs: implement memfd sealing Marc-André Lureau
2017-11-01 23:44 ` Mike Kravetz
2017-11-03 17:03 ` David Herrmann
2017-11-03 17:12 ` Mike Kravetz
2017-11-03 17:41 ` David Herrmann
2017-11-03 17:56 ` Mike Kravetz
2017-11-03 23:31 ` Mike Kravetz
2017-11-05 12:07 ` David Herrmann [this message]
2017-10-31 18:40 ` [PATCH 5/6] shmem: add sealing support to hugetlb-backed memfd Marc-André Lureau
2017-11-02 0:18 ` Mike Kravetz
2017-11-03 16:13 ` Marc-André Lureau
2017-10-31 18:40 ` [PATCH 6/6] memfd-tests: test hugetlbfs sealing Marc-André Lureau
2017-11-03 23:59 ` Mike Kravetz
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