linux-kernel - Re: [PATCH 00/28] Swap over NFS -v16

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Date:	Fri, 14 Mar 2008 16:22:27 +1100
From:	Neil Brown <neilb@...e.de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	netdev@...r.kernel.org, trond.myklebust@....uio.no,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [PATCH 00/28] Swap over NFS -v16

On Monday March 10, a.p.zijlstra@...llo.nl wrote:
> > 
> > Maybe that depends on the exact semantic of PG_emergency ??
> > I remember you being concerned that PG_emergency never changes between
> > allocation and freeing, and that wouldn't work well with slub.
> > My envisioned semantic has it possibly changing quite often.
> > What it means is:
> >    The last allocation done from this page was in a low-memory
> >    condition.
> 
> Yes, that works, except that we'd need to iterate all pages and clear
> PG_emergency - which would imply tracking all these pages etc..
> 

I don't see why you need to clear PG_emergency at all.
If the semantic is:

> >    The last allocation done from this page was in a low-memory
> >    condition.

Then you only need to (potentially) modify it's value when you
allocate it, or an element within it.

But if it doesn't fit well in the overall picture, then by all means
get rid of it.

> 
> Hmm, right. But for that purpose the names swap_* are a tad misleading.
> I remember hch mentioning this at some point. What would be a more
> suitable naming scheme so we can both use it?

One could argue that "swap" is already a misleading term.
Level 7 Unix used to do swapping.  It would write one process image
out to swap space, and read a different one in.  Moving whole
processes at a time was called swapping.
When this clever idea of only moving pages at a time was introduced (I
think in 4BSD, but possible in 2BSD and elsewhere) it was called
"demand paging" or just "paging".

So we don't have a swap partition any more.  We have a paging
partition.

But everyone calls it 'swap' and we know what it means.  I don't think
there would be a big cost in keeping the swap_ names but allowing them
to be used for occasional things other than swap.
And I suspect you would lose a lot if you tried to use a different
name that people didn't immediately identify with...

NeilBrown
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