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

[Mike Rapoport]

On Wed, Jun 28, 2017 at 11:23:32AM -0700, Prakash Sangappa wrote:
> 
> 
> 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.
 
Well, what I had in mind was using a single-process uffd monitor that will
track all the userfault file descriptors. With UFFD_EVENT_REMOVE this
process will know what areas are invalid and it will be able to process the
invalid access in any way it likes, e.g. send SIGBUS to the database
application.

If you mmap() and userfaultfd_register() only at the initialization time,
it might be also possible to avoid sending userfault file descriptors to
the monitor process with UFFD_FEATURE_EVENT_FORK.

--
Sincerely yours,
Mike.