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

[Prakash Sangappa]

On 6/28/17 6:18 AM, Mike Rapoport wrote:
> On Tue, Jun 27, 2017 at 09:01:20AM -0700, Prakash Sangappa wrote:
> > On 6/27/17 8:35 AM, Mike Rapoport wrote:
> >
> > > On Tue, Jun 27, 2017 at 09:06:43AM +0200, Michal Hocko wrote:
> > > > This is an user visible API so let's CC linux-api mailing list.
> > > >
> > > > On Mon 26-06-17 12:46:13, Prakash Sangappa wrote:
> > > >
> > > > > Any access to mapped address over holes in the file, which can occur due
> > > > > to bugs in the application, is considered invalid and expect the process
> > > > > to simply receive a SIGBUS.  However, currently when a hole in the file is
> > > > > accessed via the mapped address, kernel/mm attempts to automatically
> > > > > allocate a page at page fault time, resulting in implicitly filling the
> > > > > hole in the file. This may not be the desired behavior for applications
> > > > > like the database that want to explicitly manage page allocations of
> > > > > hugetlbfs files.
> > > > So you register UFFD_FEATURE_SIGBUS on each region tha you are unmapping
> > > > and than just let those offenders die?
> > > If I understand correctly, the database will create the mapping, then it'll
> > > open userfaultfd and register those mappings with the userfault.
> > > Afterwards, when the application accesses a hole userfault will cause
> > > SIGBUS and the application will process it in whatever way it likes, e.g.
> > > just die.
> > Yes.
> >
> > > What I don't understand is why won't you use userfault monitor process that
> > > will take care of the page fault events?
> > > It shouldn't be much overhead running it and it can keep track on all the
> > > userfault file descriptors for you and it will allow more versatile error
> > > handling that SIGBUS.
> > Co-ordination with the external monitor process by all the database
> > processes
> > to send  their userfaultfd is still an overhead.
> You are planning to register in userfaultfd only the holes you punch to
> deallocate pages, am I right?


No, the entire mmap'ed region. The DB processes would mmap(MAP_NORESERVE)
hugetlbfs files, register this mapped address with userfaultfd ones 
right after
the mmap() call.

> And the co-ordination of the userfault file descriptor with the monitor
> would have been added after calls to fallocate() and userfaultfd_register()?

Well, the database application does not need to deal with a monitor.

> I've just been thinking that maybe it would be possible to use
> UFFD_EVENT_REMOVE for this case. We anyway need to implement the generation
> of UFFD_EVENT_REMOVE for the case of hole punching in hugetlbfs for
> non-cooperative userfaultfd. It could be that it will solve your issue as
> well.

Will this result in a signal delivery?

In the use case described, the database application does not need any event
for  hole punching. Basically, just a signal for any invalid access to 
mapped
area over holes in the file.