Re: [RFC PATCH] userfaultfd: Add feature to request for a signal delivery

From: "prakash.sangappa" <prakash.sangappa@oracle.com>
To: Andrea Arcangeli <aarcange@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Christoph Hellwig <hch@infradead.org>,
	linux-api@vger.kernel.org, John Stultz <john.stultz@linaro.org>
Subject: Re: [RFC PATCH] userfaultfd: Add feature to request for a signal delivery
Date: Wed, 5 Jul 2017 15:24:14 -0700	[thread overview]
Message-ID: <a09762e7-c736-c648-2b00-3d747f75d2c4@oracle.com> (raw)
In-Reply-To: <20170704164034.GH5738@redhat.com>

On 07/04/2017 09:40 AM, Andrea Arcangeli wrote:
> On Fri, Jun 30, 2017 at 05:55:08PM -0700, prakash sangappa wrote:
>> Interesting that UFFDIO_COPY is faster then fallocate().  In the DB use case
>> the page does not need to be allocated at the time a process trips on
>> the hugetlbfs
>> file hole and receives SIGBUS.  fallocate() is called on the hugetlbfs file,
>> when more memory needs to be allocated by a separate process.
> The major difference is that with UFFDIO_COPY the hugepage will be
> immediately mapped into the virtual address without requiring any
> further minor fault. So it's ideal if you could arrange to call
> UFFDIO_COPY from the same process that is going to touch and use the
> hugetlbfs data immediately after. You would eliminate a minor fault
> that way.

Ok, we will see how it could be used in the DB use case.

>
> UFFDIO_COPY at least for anon was measured to perform better than a
> regular page fault too.
>> Regarding hugetlbfs mount option, one consideration is to allow mounts of
>> hugetlbfs inside user namespaces's mount namespace. Which would allow
>> non privileged processes to mount hugetlbfs for use inside a user
>> namespace.
>> This may be needed even for the 'min_size' mount option using which an
>> application could reserve huge pages and mount a filesystem for its use,
>> with out the need to have privileges given the system has enough hugepages
>> configured.  It seems if non privileged processes are allowed to mount
>> hugetlbfs
>> filesystem, then min_size should be subject to some resource limits.
>>
>> Mounting inside user namespace will be a different patch proposal later.
> There's no particular reason to make UFFDIO_FEATURE_SIGBUS a
> privileged op unless we want to eliminate the branch with the static
> key, so it's certainly simpler than dealing with hugetlbfs min_size
> reserves.

Ok, so, for now will not make UFFDIO_FEATURE_SIGBUS
a privileged op and not use the static key to eliminate the
branch.

> I'm positive about the UFFDIO_FEATURE_SIGBUS tradeoffs, but others
> feel free to comment.
>
> If you could make second patch to extend the selftest to exercise and
> validates UFFDIO_FEATURE_SIGBUS in anon/shmem/hugetlbfs it'd be great.

Sure, I will update the tests and send a patch.

Thanks,
-Prakash.

>
> Thanks,
> Andrea