Re: CONFIG_PREEMPT and server workloads

[Andrew Morton <akpm@osdl.org>]

Andrea Arcangeli <andrea@suse.de> wrote:
>
> > hand so we should only fall into the kmap() if the page was suddenly stolen
>  > again.
> 
>  Oh so you mean the page fault insn't only interrupting the copy-user
>  atomically, but the page fault is also going to sleep and pagein the
>  page? I though you didn't want to allow other tasks to steal the kmap
>  before you effectively run the kunmap_atomic. I see it can be safe if
>  kunmap_atomic is a noop though, but you're effectively allowing
>  scheduling inside a kmap this way.

No, we don't schedule with the atomic kmap held.  What I meant was:

	get_user(page);		/* fault it in */
	kmap_atomic(page);
	if (copy_from_user(page))
		goto slow_path;
	kunmap_atomic(page);

	...

slow_path:
	kunmap_atomic(page);
	kmap(page);
	copy_from_user(page);
	kunmap(page);


slow_path is only taken if the vm (or truncate in the case of read())
unmapped the page in that little window after the get_user().