linux-kernel - Re: [PATCH 8/9] swap_info: note SWAP_MAP

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Date:	Thu, 15 Oct 2009 23:23:24 +0100 (BST)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 8/9] swap_info: note SWAP_MAP_SHMEM

On Thu, 15 Oct 2009, KAMEZAWA Hiroyuki wrote:
> On Thu, 15 Oct 2009 01:57:28 +0100 (BST)
> Hugh Dickins <hugh.dickins@...cali.co.uk> wrote:
> 
> > While we're fiddling with the swap_map values, let's assign a particular
> > value to shmem/tmpfs swap pages: their swap counts are never incremented,
> > and it helps swapoff's try_to_unuse() a little if it can immediately
> > distinguish those pages from process pages.
> > 
> > Since we've no use for SWAP_MAP_BAD | COUNT_CONTINUED,
> > we might as well use that 0xbf value for SWAP_MAP_SHMEM.
> > 
> > Signed-off-by: Hugh Dickins <hugh.dickins@...cali.co.uk>
> 
> I welcome this!

Ah, I did wonder whether you might find some memcg use for it too:
I'm guessing your welcome means that you do have some such in mind.

(By the way, there's no particular need to use that 0xbf value:
during most of my testing I was using SWAP_MAP_SHMEM 0x3e and
SWAP_MAP_MAX 0x3d; but then noticed that 0xbf just happened to be
free, and also happened to sail through the tests in the right way.
But if it ever becomes a nuisance there, no problem to move it.)

Hugh
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