Re: [PATCH][RFC] fast file mapping for loop

[Jens Axboe <jens.axboe@oracle.com>]

On Thu, Jan 10 2008, Peter Zijlstra wrote:
> 
> On Thu, 2008-01-10 at 08:37 +0000, Christoph Hellwig wrote:
> 
> > Peter, any chance you could chime in here?
> 
> I have this patch to add swap_out/_in methods. I expect we can loosen
> the requirement for swapcache pages and change the name a little.
> 
> previously posted here:
>   http://lkml.org/lkml/2007/5/4/143
> 
> --- 
> Subject: mm: add support for non block device backed swap files
> 
> New addres_space_operations methods are added:
>   int swapfile(struct address_space *, int);
>   int swap_out(struct file *, struct page *, struct writeback_control *);
>   int swap_in(struct file *, struct page *);
> 
> When during sys_swapon() the swapfile() method is found and returns no error
> the swapper_space.a_ops will proxy to sis->swap_file->f_mapping->a_ops, and
> make use of swap_{out,in}() to write/read swapcache pages.
> 
> The swapfile method will be used to communicate to the address_space that the
> VM relies on it, and the address_space should take adequate measures (like 
> reserving memory for mempools or the like).
> 
> This new interface can be used to obviate the need for ->bmap in the swapfile
> code. A filesystem would need to load (and maybe even allocate) the full block
> map for a file into memory and pin it there on ->swapfile(,1) so that
> ->swap_{out,in}() have instant access to it. It can be released on
> ->swapfile(,0).

So this is where I don't think that's good enough, you cannot require a
full block/extent mapping of a file on setup. It can take quite some
time, a little testing I did here easily took 5 seconds for only a
couple of gigabytes. And that wasn't even worst case for that size. It
also wastes memory by populating extents that we may never read or
write.

If you look at the loop addition I did, it populates lazily as needed
with some very simple logic to populate-ahead. In practice that performs
as well as a pre-populated map, the first IO to a given range will just
be a little slower since we have to bmap() it.

Do you have plans to improve this area?

-- 
Jens Axboe