linux-kernel - Re: [PATCH v4] mm: add replace_page_cache

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Message-Id: <20110118174826.4c6d47a3.akpm@linux-foundation.org>
Date:	Tue, 18 Jan 2011 17:48:26 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Miklos Szeredi <miklos@...redi.hu>, kamezawa.hiroyu@...fujitsu.com,
	nishimura@....nes.nec.co.jp, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] mm: add replace_page_cache_page() function

On Wed, 19 Jan 2011 10:24:09 +0900 Minchan Kim <minchan.kim@...il.com> wrote:

> >
> > This is all pretty ugly and inefficient.
> >
> > We call __remove_from_page_cache() which does a radix-tree lookup and
> > then fiddles a bunch of accounting things.
> >
> > Then we immediately do the same radix-tree lookup and then undo the
> > accounting changes which we just did. __And we do it in an open-coded
> > fashion, thus giving the kernel yet another code site where various
> > operations need to be kept in sync.
> >
> > Would it not be better to do a single radix_tree_lookup_slot(),
> > overwrite the pointer therein and just leave all the ancilliary
> > accounting unaltered?
> 
> I agree single radix_tree_lookup but accounting still is needed since
> newpage could be on another zone. What we can remove is just only
> mapping->nrpages.

Well.  We only need to do inc/dec_zone_state if the zones are
different.  Perhaps the zones-equal case is worth optimising for,
dunno.

Also, the radix_tree_preload() should be unneeded.
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