From: Mike Kravetz <mike.kravetz@oracle.com>
To: Peter Xu <peterx@redhat.com>
Cc: "Axel Rasmussen" <axelrasmussen@google.com>,
"Alexander Viro" <viro@zeniv.linux.org.uk>,
"Alexey Dobriyan" <adobriyan@gmail.com>,
"Andrea Arcangeli" <aarcange@redhat.com>,
"Andrew Morton" <akpm@linux-foundation.org>,
"Anshuman Khandual" <anshuman.khandual@arm.com>,
"Catalin Marinas" <catalin.marinas@arm.com>,
"Chinwen Chang" <chinwen.chang@mediatek.com>,
"Huang Ying" <ying.huang@intel.com>,
"Ingo Molnar" <mingo@redhat.com>, "Jann Horn" <jannh@google.com>,
"Jerome Glisse" <jglisse@redhat.com>,
"Lokesh Gidra" <lokeshgidra@google.com>,
"Matthew Wilcox (Oracle)" <willy@infradead.org>,
"Michael Ellerman" <mpe@ellerman.id.au>,
"Michal Koutný" <mkoutny@suse.com>,
"Michel Lespinasse" <walken@google.com>,
"Mike Rapoport" <rppt@linux.vnet.ibm.com>,
"Nicholas Piggin" <npiggin@gmail.com>, "Shaohua Li" <shli@fb.com>,
"Shawn Anastasio" <shawn@anastas.io>,
"Steven Rostedt" <rostedt@goodmis.org>,
"Steven Price" <steven.price@arm.com>,
"Vlastimil Babka" <vbabka@suse.cz>,
linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-mm@kvack.org, "Adam Ruprecht" <ruprecht@google.com>,
"Cannon Matthews" <cannonmatthews@google.com>,
"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
"David Rientjes" <rientjes@google.com>,
"Oliver Upton" <oupton@google.com>
Subject: Re: [RFC PATCH 0/2] userfaultfd: handle minor faults, add UFFDIO_CONTINUE
Date: Mon, 11 Jan 2021 16:13:41 -0800 [thread overview]
Message-ID: <2b31c1ad-2b61-32e7-e3e5-63a3041eabfd@oracle.com> (raw)
In-Reply-To: <20210111230848.GA588752@xz-x1>
On 1/11/21 3:08 PM, Peter Xu wrote:
> On Mon, Jan 11, 2021 at 02:42:48PM -0800, Mike Kravetz wrote:
>> On 1/7/21 11:04 AM, Axel Rasmussen wrote:
>>> Overview
>>> ========
>>>
>>> This series adds a new userfaultfd registration mode,
>>> UFFDIO_REGISTER_MODE_MINOR. This allows userspace to intercept "minor" faults.
>>> By "minor" fault, I mean the following situation:
>>>
>>> Let there exist two mappings (i.e., VMAs) to the same page(s) (shared memory).
>>> One of the mappings is registered with userfaultfd (in minor mode), and the
>>> other is not. Via the non-UFFD mapping, the underlying pages have already been
>>> allocated & filled with some contents. The UFFD mapping has not yet been
>>> faulted in; when it is touched for the first time, this results in what I'm
>>> calling a "minor" fault. As a concrete example, when working with hugetlbfs, we
>>> have huge_pte_none(), but find_lock_page() finds an existing page.
>>>
>>> We also add a new ioctl to resolve such faults: UFFDIO_CONTINUE. The idea is,
>>> userspace resolves the fault by either a) doing nothing if the contents are
>>> already correct, or b) updating the underlying contents using the second,
>>> non-UFFD mapping (via memcpy/memset or similar, or something fancier like RDMA,
>>> or etc...). In either case, userspace issues UFFDIO_CONTINUE to tell the kernel
>>> "I have ensured the page contents are correct, carry on setting up the mapping".
>>>
>>
>> One quick thought.
>>
>> This is not going to work as expected with hugetlbfs pmd sharing. If you
>> are not familiar with hugetlbfs pmd sharing, you are not alone. :)
>>
>> pmd sharing is enabled for x86 and arm64 architectures. If there are multiple
>> shared mappings of the same underlying hugetlbfs file or shared memory segment
>> that are 'suitably aligned', then the PMD pages associated with those regions
>> are shared by all the mappings. Suitably aligned means 'on a 1GB boundary'
>> and 1GB in size.
>>
>> When pmds are shared, your mappings will never see a 'minor fault'. This
>> is because the PMD (page table entries) is shared.
>
> Thanks for raising this, Mike.
>
> I've got a few patches that plan to disable huge pmd sharing for uffd in
> general, e.g.:
>
> https://github.com/xzpeter/linux/commit/f9123e803d9bdd91bf6ef23b028087676bed1540
> https://github.com/xzpeter/linux/commit/aa9aeb5c4222a2fdb48793cdbc22902288454a31
>
> I believe we don't want that for missing mode too, but it's just not extremely
> important for missing mode yet, because in missing mode we normally monitor all
> the processes that will be using the registered mm range. For example, in QEMU
> postcopy migration with vhost-user hugetlbfs files as backends, we'll monitor
> both the QEMU process and the DPDK program, so that either of the programs will
> trigger a missing fault even if pmd shared between them. However again I think
> it's not ideal since uffd (even if missing mode) is pgtable-based, so sharing
> could always be too tricky.
>
> They're not yet posted to public yet since that's part of uffd-wp support for
> hugetlbfs (along with shmem). So just raise this up to avoid potential
> duplicated work before I post the patchset.
>
> (Will read into details soon; probably too many things piled up...)
Thanks for the heads up about this Peter.
I know Oracle DB really wants shared pmds -and- UFFD. I need to get details
of their exact usage model. I know they primarily use SIGBUS, but use
MISSING_HUGETLBFS as well. We may need to be more selective in when to
disable.
--
Mike Kravetz
next prev parent reply other threads:[~2021-01-12 0:39 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-07 19:04 [RFC PATCH 0/2] userfaultfd: handle minor faults, add UFFDIO_CONTINUE Axel Rasmussen
2021-01-07 19:04 ` [RFC PATCH 1/2] userfaultfd: add minor fault registration mode Axel Rasmussen
2021-01-11 11:58 ` Dr. David Alan Gilbert
2021-01-11 17:37 ` Axel Rasmussen
2021-01-11 18:09 ` Dr. David Alan Gilbert
2021-01-07 19:04 ` [RFC PATCH 2/2] userfaultfd: add UFFDIO_CONTINUE ioctl Axel Rasmussen
2021-01-11 11:43 ` [RFC PATCH 0/2] userfaultfd: handle minor faults, add UFFDIO_CONTINUE Dr. David Alan Gilbert
2021-01-11 22:42 ` Mike Kravetz
2021-01-11 23:08 ` Peter Xu
2021-01-12 0:13 ` Mike Kravetz [this message]
2021-01-12 1:49 ` Peter Xu
2021-01-12 17:37 ` Axel Rasmussen
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