Capturing all writes as fault on a memory mapped page.

[Pranay Srivastava]

On 5/28/14, Greg KH <greg at kroah.com> wrote:
> On Wed, May 28, 2014 at 06:47:24PM +0530, Pranay Srivastava wrote:
>> Hi
>>
>> I need to capture all writes on page which is memory mapped by the
>> user program.
>
> For what and what are you going to do with them once you "capture" them?
>
This is just as an exercise nothing in specific. Currently what I've
is a misc_device having a page of memory. I've made it available
through mmap to user space programs(again this is just an exercise).

> Are you trying to capture them in the kernel or in userspace?

So after a user space program has done mmap of this page to its
address space, I wanted the writes to be captured. Now if i don't set
the VM_WRITE in vm_flags then __do_fault sends sends the SEGV and if I
do set it then I'm able to get the first fault (read or write access).

>
>> I've tried setting the vm_page_prot by disabling the
>> VM_WRITE flag but even then the page fault occurs only once during the
>> first write. VM_SHARED is set to stop COW in the __do_fault.
>>
>> I was also trying to set_memory_ro(this may not be available for all
>> archs?)  however I can't do that while handling the fault since the
>> page table entries are created later after the
>> vm_operations_struct->fault has installed the page for the fault.
>>
>> I'm not having any clues on how to do this. Any help would be really
>> helpful on this.
>
> I don't think this is going to be possible, sorry, please go revisit
> the reason you think you want to do this.
>
> greg k-h
>

Actually trying to do something like if there's a write being done on
the page by a task currently then other tasks should wait( was hoping
that the fault handler code would be called for write but doesn't
happen that way).

Yes through write operation I can do with mutex but I was wondering
what if this has to be done through mmap of my misc_device then no
read/write calls involved so just wanted to see how would I stop
multiple writes to my misc_device.

Perhaps there are other methods as to how device memory can be mapped
to user space? or they aren't at all? Not really sure just reading and
trying.

-- 
        ---P.K.S