Re: [PATCH] buffermem_pages removal (5/5)

[Andrew Morton <akpm@zip.com.au>]

Christoph Hellwig wrote:
> 
> On Tue, May 21, 2002 at 10:51:19AM -0700, Andrew Morton wrote:
> > The buffermem_pages accounting is vaguely interesting because
> > it tells us how much of ZONE_NORMAL is being usefully used for
> > blockdev pagecache.  And ZONE_NORMAL utilisation is a bit of a
> > hot topic at present.
> 
> Yho sais all blockdev mapping have to stay ZONE_NORMAL?

Three trillion filesystems for which we don't have a mkfs which
access bh->b_data all over the place :(

>  If filesystems
> access them without buffer_heads there is no reason to not put the
> pages in high memory. 

They'd create a separate address_space in that case.  The blockdev
mappings are pretty unambiguously tied to ZONE_NORMAL in bdget().

> I also remember vaguely that you intend to move
> buffer_heads to high memory in the longer term..

s/buffer_heads/blockdev pages/

Yes, vaguely.  Haven't thought about it a lot.  I suspect the
present kmap() infrastructure would collapse under the load,
so surgery there would be needed first.
 
> > But the same information can be obtained on-demand by running
> > around the bdev superblock's inodes adding up nr_pages.  That
> > approach is better than the per-page atomic ops in buffer.c.
> 
> *nod*

In which case one could trivially report the number of active pages
against all superblocks.  Let's park this one until a need
is demonstrated though...

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