Re: page flags ?

[Nikita Danilov <nikita@clusterfs.com>]

Andrew Morton writes:
 > Andy Whitcroft <apw@shadowen.org> wrote:
 > >
 > >  > How many bits are spare now?  ZONETABLE_PGSHIFT hurts my brain.
 > > 
 > >  The short answer is that on 32 bit architectures there are 24 bits
 > >  allocated to general page flags, page-flags.h indicates that 21 are
 > >  currently assigned so assuming it is accurate there are currently 3 bits
 > >  free.
 > 
 > Yipes, I didn't realise we were that close.
 > 
 > We can reclaim PG_highmem, use page_zone(page)->highmem
 > 
 > We can probably reclaim PG_slab
 > 
 > We can conceivably reclaim PG_swapcache, although that stuff got ugly.
 > 
 > Would dearly love to nuke PG_reserved, but everybody's scared of that ;)
 > 
 > PG_uncached is currently ia64-only and could conceivably be moved to bit
 > 32, except there are rumours that arm might want to use it someday.
 > 
 > It's a bit irritating that swsusp uses two flags.
 > 
 > I don't see any other low-hanging fruit there.

Things like PG_uptodate and PG_error can be moved to the radix-tree tags
after checking that they are used only for pages in the mapping, which
seems to be the case.

 > --

Nikita.