Re: anon_vma RFC2

[Linus Torvalds]

On Sat, 13 Mar 2004, Rik van Riel wrote:
> 
> No, Linus is right.
> 
> If a child process uses mremap(), it stands to reason that
> it's about to use those pages for something.

That's not necessarily true, since it's entirely possible that it's just a 
realloc(), and the old part of the allocation would have been left alone.

That said, I suspect that
 - mremap() isn't all _that_ common in the first place
 - it's even more rare to do a fork() and then a mremap() (ie most of the 
   time I suspect the page count will be 1, and no COW is necessary). Most
   apps tend to exec() after a fork.
 - I agree that in at least part of the remaining cases we _would_ COW the
   pages anyway.

I suspect that the only common "no execve after fork" usage is for a few 
servers, especially the traditional UNIX kind (ie using processes are 
fairly heavy-weight threads). It could be interesting to see numbers.

But basically I'm inclined to believe that the "unnecessary COW" case is
_so_ rare, that if it allows us to make other things simpler (and thus
more stable and likely faster) it is worth it. Especially the simplicity
just appeals to me.

I just think that if mremap() causes so many problems for reverse mapping,
we should make _that_ the expensive operation, instead of making
everything else more complicated. After all, if it turns out that the
"early COW" behaviour I suggest can be a performance problem for some
(rare) circumstances, then the fix for that is likely to just let
applications know that mremap() can be expensive.

(It's still likely to be a lot cheaper than actually doing a new
mmap+memcpy+munmap, so it's not like mremap would become pointless).

			Linus
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