Re: [aarcange@redhat.com: [PATCH] fork vs gup(-fast) fix]

[Linus Torvalds <torvalds@linux-foundation.org>]

On Mon, 23 Mar 2009, Ingo Molnar wrote:
> 
> And your v2 is now:
> 
>     9 files changed, 66 insertions(+), 21 deletions(-)
> 
> ... and it is also speeding up fast-gup. Which is a marked 
> improvement IMO.

Yeah, I have no problems with that patch. I'd just suggest a final 
simplification, and getting rid of the

        mask = _PAGE_PRESENT|_PAGE_USER;
        /* Maybe the read only pte is cow mapped page. (or not maybe)
           So, falling back to get_user_pages() is better */
        mask |= _PAGE_RW;

and just doing something like

	/*
	 * fast-GUP only handles the simple cases where we have
	 * full access to the page (ie private pages are copied
	 * etc).
	 */
	#define GUP_MASK (_PAGE_PRESENT|_PAGE_USER|_PAGE_RW)

and leaving it at that.

Of course, maybe somebody does O_DIRECT writes on a fork'ed image in order 
to create a snapshot image or something, and now the v2 thing breaks COW 
on all the pages in order to be safe and performance sucks.

But I can't really say that _I_ could possibly care. I really seriously 
think that O_DIRECT and its ilk were braindamaged to begin with.

				Linus

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