Re: questions on page cache and buffers

Darin Johnson (darin@connectnet.com)
Mon, 18 Aug 1997 20:31:34 -0700 (PDT)

> For a memory/swap file system, I'm not quite clear on what you need,
> but it's a combination of the "private" mapping code (with the
> file_private_mmap vm_operations_struct so pages get sent to swap space)
> but you want your pages to be indexed in the page cache. That may
> require some tweaking of the swap_in code.
>
> I think you just want everything in the page cache, some completely
> anonymous pages for metadata, and some indexed pages for file data.
> Doing that, 99% of file accesses will be hits in the page cache and
> you don't have to worry about I/O to them at all.
> --
> -Colin

Looking closer, how to do what I want seems murky. Basically, I
want a memory based filesystem, for a 'tempfs' style thing.

The snags are:
The page cache discards pages without having a chance to write
them anywhere (it assumes the coming from a real device, and
that the pages are all temporary pages that can be discarded).

Tracking this down, I don't see any connection between writing
to a page and it ever getting written out. The generic_write_page
might be OK, but noone but nfs/autofs even uses that. Looking at
minixfs, it uses generic_read_file and minix_write_file, and
minix_write_file seems to operate on buffers, not pages.

So now it seems, if I have read_page just return one of my own
pages; the system can discard that page behind my back. But if
I return a copy of a page, then that's a waste of memory. (maybe
just discard my copy if it's on disc)

Hmm, maybe I should use NFS as my guide, since it doesn't use
a device underneath.