Re: [patch] mm: mremap correct rmap accounting

[Hugh Dickins <hugh@veritas.com>]

On Mon, 29 Jan 2007, Linus Torvalds wrote:
> 
> How about:
>  - the current code has worked so far, so there is no way in hell I'll 
>    take a patch for some odd-ball architecture for a theoretical problem 
>    that nobody else cares about for 2.6.20 *anyway* and hasn't been 
>    reported until now (considering that it's apparently been around since 
>    rmap went in).
> 
>  - somebody who cares would explore trying to make page_mapcount work like 
>    page_count, and see if this is a viable approach.
> 
>  - at worst, we can do the Andrew thing, and just forget about the 
>    ZERO_PAGE() multi-page thing. It may well be that even MIPS people 
>    don't care any more, if the particular uarch version that was helped 
>    most isn't very common any more (we can always hope..)

I'm currently thinking the right answer will be to keep the MIPS
multiple ZERO_PAGEs, for their performance benefit, but ditch the
way MIPS moves from one ZERO_PAGE to another in move_pte, trying
desperately to retain that performance benefit across mremap.

It never used to do that, it was something Nick noticed and put
into 2.6.14.  I've the ghost of a memory that at that time he
had some code which was actually relying on ZERO_PAGE(vaddr)
always being the expected ZERO_PAGE for that vaddr.
I'm about to search my old mailboxes.

Hugh

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