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

[prakash.sangappa]

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