So what the heck is anonymous memory

As part of some work I've been doing I've had to talk a lot about anonymous memory. So I thought I'd write it down while I remember what its all about. anonymous memory is memory that is managed by segvn but is not really directly associated with a file. It's used for things like process stacks, heap, or COW (copy on write) pages. A good example of this is if you fork a process. All the addresses in the second process actually map back to the same bits of physical memory (the same pages). However if youre child process was then to do something different with the memory (eg. the child went off and manipulated an array in memory) the VM subsytem would copy those pages and change the mappings in the child process to point to the new pages. This new memory would be anonymous memory, and the child process would merrily make the changes to the array, unaware it now had new "physical" memory it was talking to. In a bit more detail, you've probably heard people talking about "anon_maps" and "vnode, offsets". Or at least if you're interested in VM and have been trawling the code you probably have. So I'll try and put a simplistic view of what they mean. Typically when you mmap a file, you are give it a vnode, offset and length. Segvn manages mmaped files and stores this in its private data structure. So when you walk through the address space of the process looking for a virtual address, you'll see that the segment contains that address and be able to map it to the vnode and where in that file using the offset. As mentioned above anonymous memory is not associated with files, but with perhaps swap (you'd get to uses anon memory if you mmaped /dev/zero BTW). So the purpose of anonymous memory or the anon layer is to fake up a vnode and offset for segvn to find the data on a swap device (actually by going through swapfs which I haven't yet looked at). The core of the anon layer is the anon_map structure. This is stored in the segvn_data structure and points you to the anon_hdr for this segment. Each anon_hdr is linked to an array on anon structures which are the swapfs implementation of how to find a vnode and offset on a swap file system (or it might be on a real swap device if you bypass swapfs, I think I'll need another blog entry for swapfs after this). So after some swapfs magic we can find the backing store for this anonmap. You might have noticed a bit if handwaiving in the middle there. What was that about an array of anon structures? Why do we need one of them? Well each anon strucure represents a page of memory. Our segment may be more than one page in size (try pmap $$ and you may well see some anon segments of more than 8K on sparc) so would need more than one of these anon structure to describe it. So we have this array. Technorati Tag:
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Comments:

Thanks, this was really helpful. You should put it up on wikipedia.

Posted by Sameer on December 25, 2005 at 09:35 PM GMT #

Ditto, I've been going in circles trying to find out what the heck what an anon page is (and why there are so many of them). This entry clears that up succinctly. Bravo.

Posted by Roger Warner on November 22, 2006 at 11:34 AM GMT #

Awesome!!! thanks a lot

Posted by Anirudh tomer on February 11, 2011 at 03:07 AM GMT #

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Chris W Beal

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