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

On 06/29/2017 03:46 AM, Mike Rapoport wrote:

On Wed, Jun 28, 2017 at 11:23:32AM -0700, Prakash Sangappa wrote:

[...]

quoted

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.

Use of a monitor process is also an overhead for the database.

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.

The new processes are always exec'd in the database case and these
processes could be mapping different files. So, not sure if
UFFD_FEATURE_EVENT_FORK will be useful.  Also, it may not be one
process spawning the other new processes.

--
Sincerely yours,
Mike.

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