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Date: Tue, 18 Jan 2011 17:48:26 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Minchan Kim <minchan.kim@gmail.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>, kamezawa.hiroyu@jp.fujitsu.com,
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Subject: Re: [PATCH v4] mm: add replace_page_cache_page() function
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On Wed, 19 Jan 2011 10:24:09 +0900 Minchan Kim <minchan.kim@gmail.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.