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

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Date:   Tue, 4 Jul 2017 18:40:34 +0200
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     prakash sangappa <prakash.sangappa@...cle.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        linux-api@...r.kernel.org, John Stultz <john.stultz@...aro.org>
Subject: Re: [RFC PATCH] userfaultfd: Add feature to request for a signal
 delivery

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.

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.

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.

Thanks,
Andrea