Re: [PATCH] VFS: Pagecache usage optimization on pagesize != blocksize environment

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 21 May 2008 00:19:30 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:

> 
> I guess a suitable fix might be to implement the above using a new
> address_space_operations callback:
> 
> 	if (PagePrivate(page) && aops->is_partially_uptodate) {

This is a bit presumptuous.  A filesytem could in theory be able to
work out if a section of a page is uptodate without necessarily
maintaining per-page metadata at page.private.

So one really should just do

	if (aops->is_partially_uptodate)

however it's hard to conceive how a filesystem could sanely do this
_without_ putting data at page.private, so the PagePrivate() test could
perhaps be retained as a microoptimisation, as long as it is suitably
commented.

otoh, non-zero aops->is_partially_uptodate might well be less common
than non-zero PagePrivate(), so perhaps these should be tested in the
other order.  Not sure.

> 		if (aops->is_partially_uptodate(page, desc, offset))
> 			<OK, we can copy the data>
> 
> then implement a generic_file_is_partially_uptodate() in fs/buffer.c

Make that block_is_partially_uptodate().

> and wire that up in the filesystems.
> 
> Note that things like network filesystems can then implement this also.