On Thu, Jan 10 2008, Peter Zijlstra wrote: > > On Thu, 2008-01-10 at 10:49 +0100, Jens Axboe wrote: > > 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? > > Nope, for swap it _must_ be there, there is just no way we can do block > allocation on swapout. I appreciate that fact, just saying that we could have more flexibility for other uses. > That said, the swapfile() interface can be used to pre-populate the > extend/block mapping, and when using swap_(in/out) without it, it can be > done lazily. I think the interface is too simple, I would much rather have a way to dig into the file mappings and be allowed to request population and so on. Without that, it can't be used for eg loop. -- Jens Axboe - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
