Re: [RFC] Remove unswappable anonymous pages off the LRU

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

On Thu, 15 Feb 2007 18:34:12 -0800 (PST)
Christoph Lameter <clameter@sgi.com> wrote:

> On Thu, 15 Feb 2007, Andrew Morton wrote:
> 
> > Guys, this page-flag problem is really serious.  -mm adds PG_mlocked and
> > PG_readahead and the ext4 patches add PG_booked (am currently fighting the
> > good fight there).  There's ongoing steady growth in these things and soon
> > we're going to be in a lot of pain.
> 
> Well is it possible to restrict some of the features to 64 bit only? There 
> we have lots of page flags.

We discussed that a while back and iirc ia64 has gone and gobbled most of
the upper 32bits.  Someone went and added some ascii art around the
PG_uncached definition but it is incomprehensible.  It seems to claim that
ia64 has gone and used all 32 bits, dammit.  If so, some adjustments to
ia64 might be called for.

> One additional measure that may be possible is to have a page type field
> (maybe 3 bits long) that would consolidate a series of page flags that 
> cannot occur together. But then we have issues with the atomicity of 
> updates to that field.
> 
> F.e.
> 
> page_type = { SLAB, LRU, MLOCK, RESERVED, BUDDY, <add 3 more types here> }

Yeah, maybe.  There doesn't seem to be a lot of room for that though - a
lot of those flags are quite independent and can occur simultaneously.

Maybe PageSwapCache can be worked out by other means.

The two swsusp bits can be removed: they're only needed at suspend/resume
time and can be replaced by an external data structure.

I still reckon there must be a way to avoid PG_buddy but Martin put up
stiff-and-squealy resistance when I resisted the addition of that.

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