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@gmail.com> wrote:

Hi Riya,

On Mon, Jan 12, 2015 at 2:08 AM, riya khanna <riyakhanna1983@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.

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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

With shared memory mappings, once A and B both map the same memory address/file they see the same contents A [0xaaaaaaaa] - maps - to -> [0xAAAAAAAA] B [0xbbbbbbbb] - maps - to -> [0xAAAAAAAA] However, if A changes (update) the mappings to point to a different memory area/file offset, B sees different contents A [0xaaaaaaaa] - maps - to -> [0xBBBBBBBB] Is there a way to force update the mappings of B, so that B too sees contents at 0xBBBBBBBB? Could this be done, by deleting the shmem backing and making the fault handler update the mappings? On Mon, Jan 12, 2015 at 10:50 AM, <Valdis.Kletnieks@vt.edu> wrote:

On Mon, 12 Jan 2015 07:34:41 -0600, riya khanna said:

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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.

Umm..B should be seeing what A does *already*. That's the whole *point* of shared mmap spaces.

And what problem are you trying to solve by doing this, anyhow? There's probably a better approach (for starters, have A set up the mappings, then fork B which gets the mappings all set up from the start).

Hi Riya, On Mon, Jan 12, 2015 at 10:01 AM, riya khanna <riyakhanna1983@gmail.com> wrote:

With shared memory mappings, once A and B both map the same memory address/file they see the same contents A [0xaaaaaaaa] - maps - to -> [0xAAAAAAAA] B [0xbbbbbbbb] - maps - to -> [0xAAAAAAAA]

However, if A changes (update) the mappings to point to a different memory area/file offset, B sees different contents A [0xaaaaaaaa] - maps - to -> [0xBBBBBBBB]

Is there a way to force update the mappings of B, so that B too sees contents at 0xBBBBBBBB? Could this be done, by deleting the shmem backing and making the fault handler update the mappings?

No you can't do that. This is an easily solvable problem with user-space code. What you are saying is that if the page is dirty for process A you want process B to automatically update it's copy. This just won't happen and certainly not the way you want it to happen. You need to design this properly. Better yet take some transaction id over the device which_device_should_tell you either by doing some ioctl or other way. Have some wrappers like sync_mapped_memory(device_fd) etc that will first sync the process's copy before doing anything. What you are saying of happening automatically would mean that a write at one place would trigger a fault in another process's address space but that's not how it would work and it's not good design. care to tell more about the problem? maybe I can help

On Mon, Jan 12, 2015 at 10:50 AM, <Valdis.Kletnieks@vt.edu> wrote:

...

On Mon, 12 Jan 2015 07:34:41 -0600, riya khanna said:

...

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.

Umm..B should be seeing what A does *already*. That's the whole *point* of shared mmap spaces.

And what problem are you trying to solve by doing this, anyhow? There's probably a better approach (for starters, have A set up the mappings, then fork B which gets the mappings all set up from the start).

-- ---P.K.S