Re: [PATCH v4] mm: add replace_page_cache_page() function

[Minchan Kim]

On Wed, Jan 19, 2011 at 10:48 AM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, 19 Jan 2011 10:24:09 +0900 Minchan Kim <minchan.kim@xxxxxxxxx> 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.

Agree.
In summary, optimization points are following as.

1) remove radix_tree_preload
2) single radix_tree_lookup_slot and replace radix tree slot
3) page accounting optimization if both pages are in same zone.

I hope we mm guys optimize the above things with TODO.
(Except freepage issue I mentioned.)
So I want Miklos resend the patch with solving freepage issue and NOTE
of TODO in description or comment.

-- 
Kind regards,
Minchan Kim
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