On 15-Nov-05, at 3:58 AM, John Giacomoni wrote:
On Nov 14, 2005, at 1:57 PM, Scott Long wrote:
John Giacomoni wrote:
I am in need of a way to share memory between kernel space and
possibly
multiple different user-space processes for an extended period of
time.
This memory would need to be a single unpageable region.
I am using the vm routines as cribbed from mmap, however I'd like
the
address spaces to be viewed as the same regardless of which
process I'm
in to avoid swizzling pointers as I'm storing data structures in the
shared memory region.
I imagine I'd need to find a way to expose part of the kernel
address
space to user space to accomplish this.
Is there a way to do this?
thanks
John G
If you get this working then it'll be very useful for the syspage
support that was talked about recently.
The kernel can access addresses in the user space so long as they
are wired and won't cause a fault. Thus I imagine that you
only need to allocate the memory, wire it, mark it with the
appropriate
page permissions, and reserve a user address range for it in the
process map. I'd look at the process exec path in the kernel for
places to hook in. The only other trick then is how to let the user
process know the address for this magic region. An easy way would be
to store it in a sysctl that can be read at runtime. A harder way
would
be to have the kernel dummy up an elf segment in the image activator
code that the dynamic linker could read and put into a global
variable
for the program to access.
Scott
Which thread/list was the syspage support discussed?
(I did a quick search in my freebsd-hackers mailbox and didn't find
it)
As for my problem I'm not sure that the kernel being able to access
userpages
is quite what is needed. The first reason is that if there are two
processes
trying to map the same region it would be problematic if one of
them had
allocated something in the same area of the heap as the shared
memory region
Typically, the address range for heap is different from that of
shared memory segment. Im not sure of freebsd -but for AIX what Im
saying is true. So, unless you have allocated another shared memory
object, MAP_FIXED should suffice to get the same address across user
processes.
resides. This suggests that to avoid swizzling the region needs to
be in an
address region where the user-space processes will never allocate
in (kernel).
The other reason is that the kernel code needs to be able to
function using
the shared memory without a user-space process being attached at
all times.
So the question is, is it possible to mark wired kernel pages as
being accessible
to a user-space process without triggering a context switch?
How about you bring in a kernel process to access the mmap'ed file/
shmid?
regards
-kamal
John G
--
[EMAIL PROTECTED]
University of Colorado at Boulder
Department of Computer Science
Engineering Center, ECCR 1B05
430 UCB
Boulder, CO 80303-0430
USA
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Kamal R. Prasad
UNIX systems consultant
http://www.kamalprasad.com/
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