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 [off-list ref] wrote:

Hi Riya,


On Mon, Jan 12, 2015 at 2:08 AM, riya khanna <riyakhanna1983@gmail.com
<javascript:;>> wrote:

quoted

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.

quoted

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?

quoted

Thanks for your help!

-Riya

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        ---P.K.S

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