Re: remove zero_page (was Re: -mm merge plans for 2.6.24)

[Hugh Dickins]

On Tue, 9 Oct 2007, Nick Piggin wrote:
> 
> The commit b5810039a54e5babf428e9a1e89fc1940fabff11 contains the note
> 
>   A last caveat: the ZERO_PAGE is now refcounted and managed with rmap
>   (and thus mapcounted and count towards shared rss).  These writes to
>   the struct page could cause excessive cacheline bouncing on big
>   systems.  There are a number of ways this could be addressed if it is
>   an issue.
> 
> And indeed this cacheline bouncing has shown up on large SGI systems.
> There was a situation where an Altix system was essentially livelocked
> tearing down ZERO_PAGE pagetables when an HPC app aborted during startup.
> This situation can be avoided in userspace, but it does highlight the
> potential scalability problem with refcounting ZERO_PAGE, and corner
> cases where it can really hurt (we don't want the system to livelock!).
> 
> There are several broad ways to fix this problem:
> 1. add back some special casing to avoid refcounting ZERO_PAGE
> 2. per-node or per-cpu ZERO_PAGES
> 3. remove the ZERO_PAGE completely
> 
> I will argue for 3. The others should also fix the problem, but they
> result in more complex code than does 3, with little or no real benefit
> that I can see. Why?

Sorry, I've no useful arguments to add (and my testing was too much
like yours to add any value), but I do want to go on record as still
a strong supporter of approach 3 and your patch.

Hugh
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