Shared mapping

[riya khanna]

Suppose A and B have mapped the same physical memory or shmem file. I want
a way to make process A forcefuly revoke/remap the existing shared memory
mappings in process B, so that B sees whatever A does.

On Monday, January 12, 2015, Pranay Srivastava <pranjas at gmail.com> wrote:

> Hi Riya,
>
>
> On Mon, Jan 12, 2015 at 2:08 AM, riya khanna <riyakhanna1983 at gmail.com
> <javascript:;>> wrote:
> > Hi,
> >
> > Is there a way, w.r.t memory contents, to make a process B see what
> > process A sees through shared memory mappings?
> >
> > Suppose Process A has mapped a device file (say F) at address
> > 0xaaaaaaaa in its address space. Is it possible for process B to see
> > the same device file F contents as process A via shared memory
> > mappings? If mapped in process B's address space, would process B
> > continue to to see the old contents if process A were to map a new
> > device file (say N) at the same address 0xaaaaaaaa in its address
> > space or do they have independent mappings?
>
> It depends on the driver mapping that specific memory address. If it
> so chooses that every process trying to map that address needs to have
> an independent page then it'll do that. But normally if the driver
> allows mmapping device then it'll be a separate copy.
>
> >
> > So basically I'm trying to understand if there could be virtual to
> > virtual mappings (e.g. process B mapping process A's memory, so that B
> > sees whatever A sees)?
>
> This means you are getting two separate pages when you mmap the device?
>
> Normally your driver won't give you same page since once a page is
> mapped simple writes(like assignments etc) won't trigger any fault
> which might give the driver a chance to do proper locking semantics
> when both processes are writing at same time using just an assignment.
>
> What you can do is create a shared mapping between the two processes A
> and B then fill that mapping from whoever wins the race to update this
> shared mapping between them. You can add some information if the
> information needs to be updated again in this shared region between A
> and B.
>
> Can you tell more about your problem?
> >
> > Thanks for your help!
> >
> > -Riya
> >
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>
>
>
> --
>         ---P.K.S
>
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