MAP_NOZERO revisited

[Arun Sharma <asharma@fb.com>]

A few years ago, Davide posted patches to address clear_page() showing 
up high in the kernel profiles.

http://thread.gmane.org/gmane.linux.kernel/548928

With malloc implementations that try to conserve the RSS by madvising 
away unused pages that are dirty (i.e. faulted in), we pay a high cost 
in clear_page() if that page is needed later by the same process.

Now that we have memcgs with their own LRU lists, I was thinking of a 
MAP_NOZERO implementation that tries to avoid zero'ing the page if it's 
coming from the same memcg.

This will probably need an extra PCG_* flag maintaining state about 
whether the page was moved between memcgs since last use.

Security implications: this is not as good as the UID based checks in 
Davide's implementation, so should probably be an opt-in instead of 
being enabled by default.

Comments?

  -Arun

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