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

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

On Thu, 15 Feb 2007 17:40:09 -0800
Martin Bligh <mbligh@mbligh.org> wrote:

> Andrew Morton wrote:
> > On Thu, 15 Feb 2007 13:05:47 -0800 (PST)
> > Christoph Lameter <clameter@sgi.com> wrote:
> > 
> >> If we do not have any swap or we have run out of swap then anonymous pages
> >> can no longer be removed from memory. In that case we simply treat them
> >> like mlocked pages. For a kernel compiled CONFIG_SWAP off this means
> >> that all anonymous pages are marked mlocked when they are allocated.
> > 
> > It's nice and simple, but I think I'd prefer to wait for the existing mlock
> > changes to crash a bit less before we do this.
> > 
> > Is it true that PageMlocked() pages are never on the LRU?  If so, perhaps
> > we could overload the lru.next/prev on these pages to flag an mlocked page.
> > 
> > #define PageMlocked(page)	(page->lru.next == some_address_which_isnt_used_for_anwything_else)
> 
> Mine just created a locked list. If you stick them there, there's no
> need for a page flag ... and we don't abuse the lru pointers AGAIN! ;-)

I don't think there's a need for a mlocked list in the mlock patches:
nothing ever needs to walk it.

However this might be a good way of solving the someone-did-a-swapon
problem for this anon patch.

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.

> Suspect most of the rest of my patch is crap, but that might be useful?

wordwrapped, space-stuffed and tab-replaced.  The trifecta!

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