On Mon, Jul 09, 2007 at 05:59:47PM -0700, Christoph Lameter wrote:

On Tue, 10 Jul 2007, Nick Piggin wrote:

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Hmmm.... I did not notice that yet but then I have not done much work 
there.

Notice what?

The bad code for the buffer heads.

Oh. Well my first mail in this thrad listed some of the problems
with them.

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- A real "nobh" mode. nobh was created I think mainly to avoid problems
  with buffer_head memory consumption, especially on lowmem machines. It
  is basically a hack (sorry), which requires special code in filesystems,
  and duplication of quite a bit of tricky buffer layer code (and bugs).
  It also doesn't work so well for buffers with non-trivial private data
  (like most journalling ones). fsblock implements this with basically a
  few lines of code, and it shold work in situations like ext3.

Hmmm.... That means simply page struct are not working...

I don't understand you. jbd needs to attach private data to each bh, and
that can stay around for longer than the life of the page in the pagecache.

Right. So just using page struct alone wont work for the filesystems.

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There are no changes to the filesystem API for large pages (although I
am adding a couple of helpers to do page based bitmap ops). And I don't
want to rely on contiguous memory. Why do you think handling of large
pages (presumably you mean larger than page sized blocks) is strange?

We already have a way to handle large pages: Compound pages.

Yes but I don't want to use large pages and I am not going to use
them (at least, they won't be mandatory).

 

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Conglomerating the constituent pages via the pagecache radix-tree seems
logical to me.

Meaning overhead to handle each page still exists? This scheme cannot 
handle large contiguous blocks as a single entity?

Of course some things have to be done per-page if the pages are not
contiguous. I actually haven't seen that to be a problem or have much
reason to think it will suddenly become a problem (although I do like
Andrea's config page sizes approach for really big systems that cannot
change their HW page size).