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SubjectRe: Status of AIO
On Mon, Mar 06, 2006 at 08:39:15PM -0500, Benjamin LaHaise wrote:
> On Mon, Mar 06, 2006 at 04:51:29PM -0800, David S. Miller wrote:
> > I think any such VM tricks need serious thought. It has serious
> > consequences as far as cost especially on SMP. Evgivny has some data
> > that shows this, and chapter 5 of Networking Algorithmics has a lot of
> > good analysis and paper references on this topic.
>
> VM tricks do suck, so you just have to use the tricks that nobody else
> is... My thinking is to do something like the following: have a structure
> to reference a set of pages. When it is first created, it takes a reference
> on the pages in question, and it is added to the vm_area_struct of the user
> so that the vm can poke it for freeing when memory pressure occurs. The
> sk_buff dataref also has to have a pointer to the pageref added. Now, the
> trick to making it useful is as follows:
>
> struct pageref {
> atomic_t free_count;
> int use_count; /* protected by socket lock */
> ...
> unsigned long user_address;
> unsigned long length;
> struct socket *sock; /* backref for VM */
> struct page *pages[];
> };
[...]
>
> It's probably easier to show this tx path with code that gets the details
> right.

This somehow resembles the scatter-gatter lists already used in some
subsystems such as the SCSI sg driver.

BTW you have to make these pages Copy-On-Write before this procedure
starts because you wouldn't want it to accidently fill the zero page,
i.e. the VM will have to supply a unique set of pages otherwise it
messes up.

--
Dan Aloni
da-x@monatomic.org, da-x@colinux.org, da-x@gmx.net, dan@xiv.co.il
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