On 5/27/13 4:56 PM, Alfred Perlstein wrote:
On 5/27/13 4:38 PM, Luigi Rizzo wrote:
Hi,
say a process P1 wants to use the kernel to copy the content of a
buffer SRC (in its user address space) to a buffer DST (in the
address space of another process P2), and assume that P1 issues the
request to the kernel when P2 has already told the kernel where the
data should go:
P1 P2
+------+ +--------+
| SRC | | DST |
+--v---+ +--^-----+
--------|------------------------|----------
| | kernel
| ^
| |
| +--------+ |
+----->| tmpbuf +--------+
copyin| | copyout
P1 ctx+--------+ P2 ctx
I guess the one above is the canonical way: P1 does a copyin() to a
temporary buffer, then notifies P2 which can then issue or complete
a syscall to do a copyout from tmpbuf to DST in P2's context.
But I wonder, is it possible to do it as follows: P2 tells the kernel
where the data should go (DST); later, P1 issues a system call and
through a combined "copyinout()" moves data directly from SRC to DST,
operating in the context of P1.
| copyinout() ? |
+------------>-----------+
issued by P1
Is this doable at all ? I suspect that "tell DST to the kernel"
might be especially expensive as it needs to pin the page
so it is accessible while doing the syscall for P1 ?
(the whole point for this optimization is saving the extra
copy through the buffer, but it may be pointless if pinning
the memory is more expensive than the copy)
I suspect you'll want to use something like vslock(9) and sf_bufs.
Have a look at vm/vm_glue.c -> vslock() vm_imgact_hold_page().
On amd64, I *think* mapping an sfbuf or if you are really evil you can
optimistically wire the page in the vm (cheap). If it's present then
you can just use the direct map to access it. However, if it's not
present, then fall back to another method, or maybe just fault it in
(which will have to happen anyhow) and then retry.
Sounds like a cool project!
-Alfred
Oh, one other thing.. look at the pipe code. It used to do what you
suggest, I think however it was driven by the READER pinning the
WRITER's address space and doing a direct copy. However it may not be
optimized for NOT-mapping into kva as I suggested doing.
-Alfred
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