Date: Fri, 16 May 2008 09:34:06 -0500 (CDT) From: Mark Tinguely <tinguely@casselton.net> To: teemu@rinta-aho.org Cc: freebsd-hackers@freebsd.org Subject: Re: copy-on-write anonymous memory? Message-ID: <200805161434.m4GEY6N3082155@casselton.net> In-Reply-To: <482C4DF3.8030709@rinta-aho.org>
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Teemu Rinta-aho wrote (some edits):
> I have created a kernel module that stores references to memory objects.
> I.e. when a process makes a syscall to the module, it will create a
> snapshot of the memory area, and after that the writes from the process to
> that memory area should create a shadow object. The next syscall should
> again store a pointer to the current topmost shadow object and then the
> next write creates yet another shadow object. Etc... When the snapshots
> are removed, the shadow chains may collapse normally.
>
> Here's an illustration of what I want (first syscall OK, second one not):
>
> * Legend: U/u = userspace K/k = kernel
> *
> * U:vm_map_entry_u -> object
> * ||
> * SYSCALL
> * ||
> * \/
> * U:vm_map_entry_u -> object_shadow -> object
> * /
> * K:vm_map_entry_k ----------------
> * ||
> * SYSCALL
> * ||
> * \/
> * U:vm_map_entry_u -> object_shadow -> object_shadow -> object
> * / /
> * K:vm_map_entry_k ---------------- /
> * K:vm_map_entry_k --------------------------------
>
> Now, the problem is that the first snapshot works as it should. However,
> the second one doesn't, and the write goes to the one and same shadow
> object, even if I restore MAP_ENTRY_COW and MAP_ENTRY_NEEDS_COPY manually
> in my handler function which is storing the snapshot.
>
> Any ideas?
Usually, a fork() creates the inheritance between parent and child COW memory
space. Start:
vm_map_entry -> object_shadow -> object
fork():
vm_map_entry -> object_shadow -\
|-> (object_shadow*) -> object
vm_map_entry -> object_shadow -/
This is slightly different from your description/drawing, in the way changes
are inherited; for example: process 1 is created, process 1 writes page 0.
process 2 is created. process 1 writes p1 (or p0 again). Your
description/drawing implies that process 2 see this change from process 1.
You are not forking over a COW memory area. Sounds like the syscall will have
manually create the inheritance. You can manually link the object_shadows
the way you want to get the desired inheritance. Process removals should
collapse the shadows automatically.
Matt Dillion wrote a brief VM description
(http://www.freebsd.org/doc/en_US.ISO8859-1/articles/vm-design/).
The book, "The Design and Implementation of the 4.4BSD Operating System"
is another great reference.
Mark Tinguely
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