Subject: Re: tmpfs: Storing file contents
To: Julio M. Merino Vidal <email@example.com>
From: Bill Studenmund <firstname.lastname@example.org>
Date: 08/12/2005 09:24:01
Content-Type: text/plain; charset=us-ascii
On Fri, Aug 12, 2005 at 12:27:56PM +0200, Julio M. Merino Vidal wrote:
> Hi all,
> during the last days, I've implemented very clumsy read/write vnode
> operations for tmpfs, just to familiarize myself with uiomove and the
> like (they are not in the CVS). I've also been reading the first chapters
> of Cranor's UVM dissertation to see how to manage anonymous
> memory, although I still have many doubts.
> The thing is that I don't know how to store file contents in memory
> to comply to the following requirements:
> - Use pageable (unwired) memory (i.e., anonymous memory, right?).
> - Avoid multiple copies of the same data on memory.
You can do this by making the VM and read/write interfaces use the same=20
storage. If you use the normal read & write routines that use=20
memory-mapped pages for i/o, you're fine.
> - Be careful to not introduce a lot of overhead on the memory manager.
> I see two possibilities as regards how to manage each file:
> - One of them is to allocate space page by page (just as a file-system
> allocates blocks) and attach these pages to the files needing them.
> The fact that they are pages does not matter in the idea: we'd simply
> be dealing with a structure whose size matches a page, but that's all.
> That is, keep a sorted list (or another structure with better random
> access times) of pages (blocks) that contain the file. This'd be like
> having a pool of fixed size structures, but over pageable memory.
> In order to do this, I'd be nice if I could allocate a big address space
> and allocate/deallocate individual pages easily.
My concern with this is that you now have files coming out of a backing=20
store. So you've recreated a traditional file system, just with the=20
backing in pageable memory.
> - Have an independent virtual address space for each file, backed by
> anonymous memory, so that reads and writes are trivial: just read
> and write from memory at a specific offset within the address
> space. Page mapping could be automatic upon faults (extra steps
> could be needed to unmap unused pages to avoid swap leaks).
> I don't know if this implementation is possible at all, or if it could
> cause too much overhead on UVM... but if possible, it'd simplify
> things a lot.
I _think_ this is the right way to handle things, though the Chucks and=20
Jason will know better. Maybe your #1 way is better, but I like this one.=
> - Of course, there may be other better possibilities, but I can't see
> them with my current knowledge.
I think those are the two main ideas. The differentiator would be access=20
abilities and meory use. Memory use in the extra structures needed for=20
each method (more aobjs vs whatever maps a file to/from the blocks in the=
one aobj of option 1), and access abilities for the locking (each aobj=20
will be lockable on its own, but there will need to be some shared locking=
on option 1).
> The problem is that... aside not knowing which approach should I
> follow, I don't know how to code them as regards memory
> management. I've read the uvm(9) manpage and searched for
> usage examples of its functions within the kernel. Unfortunately,
> I can't find much: just some calls in process management and
> SHM... but these don't seem to be what I need (or at least I can't
> find the appropriate examples).
> Some of my doubts are...
> - Do I have to keep an aobj for each file? If so, aobj's are created
> with uao_create, right? Which size should they have (as it's not
> known beforehand)?
Well, I don't think you HAVE to keep an aobj for each file, I just think=20
it's the best idea. :-) I'm not sure about what size it should be, but=20
you'll have to be able to grow it anyway...
> - Once I have an aobj, how do I map space within it? Maybe I have
> to use uvm_map, but I'm afraid that calling that function to request
> single pages could introduce a lot of overhead... Or can it be
> done in a more automated way, as I described in the second
> approach above?
> - When some people mentioned that I should avoid having multiple
> copies of the same data in memory, they were referring to one
> copy of the data managed by the filesystem and another one
> stored inside the vnode's uobj, right? If so, can't this be avoided
> by having a getpages operation that simply loans pages from the
> filesystem to the vnode (thus just keeping one real copy)?
Exactly. That's the point of UBC.
> I'm sorry for so many questions, but I'm really lost in this area.
> I will appreciate any suggestion, pointers to documentation or code,
> or even detailed explanations of the process I should follow (e.g.,
> which is the basic idea to store the files, which functions should I
> look at, etc.)...
Hope this helps.
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