Re: [PATCH] Don't needlessly dirty mlocked pages when initially faulting them in.

[Suleiman Souhlal <ssouhlal@FreeBSD.org>]

On Jul 26, 2007, at 11:33 PM, Peter Zijlstra wrote:

> On Thu, 2007-07-26 at 17:23 -0700, Andrew Morton wrote:
> > On Thu, 26 Jul 2007 16:52:44 -0700 Suleiman Souhlal  
> > <ssouhlal@FreeBSD.org> wrote:
> >
> > > make_pages_present() is dirtying mlocked pages if the VMA is  
> > > writable, even
> > > though it shouldn't, by telling get_user_pages() to simulate a  
> > > write fault.
> > >
> > > A simple way to test this is to mlock a multi-GB file, and then  
> > > sync.
> > > The sync will take a long time.
> >
> > ugh, how bad of us.
> >
> > > As far as I can see, it should be safe to just not simulate a  
> > > write fault.
> >
> > We pass in "write=1" to force a COW.  This is because we want to  
> > do all
> > that memory allocation at mlock()-time, not later on, when the app  
> > writes
> > to the page.
>
> <snip patch>
>
> > So something sterner will need to be done.  I guess the  
> > write_access arg to
> > handle_mm_fault() would need to become a three-value thing.
>
> That would be most painfull. Can't we simply set write=0 for shared
> mappings? Those won't have COW to break and are the onces that do
> requires writeback. Anonymous and private mappings do COW but will  
> never
> writeback and are thus save to touch with write=1.
>
> How about something like this:
>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
>  mm/memory.c |    7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> Index: linux-2.6/mm/memory.c
> ===================================================================
> --- linux-2.6.orig/mm/memory.c
> +++ linux-2.6/mm/memory.c
> @@ -2716,7 +2716,12 @@ int make_pages_present(unsigned long add
>  	vma = find_vma(current->mm, addr);
>  	if (!vma)
>  		return -1;
> -	write = (vma->vm_flags & VM_WRITE) != 0;
> +	/*
> +	 * We want to touch writable mappings with a write fault in otder
> +	 * to break COW, except for shared mappings because these don't COW
> +	 * and we would not want to dirty them for nothing.
> +	 */
> +	write = (vma->vm_flags & VM_WRITE|VM_SHARED) == VM_WRITE;
>  	BUG_ON(addr >= end);
>  	BUG_ON(end > vma->vm_end);
>  	len = DIV_ROUND_UP(end, PAGE_SIZE) - addr/PAGE_SIZE;

Oops, I completely forgot about this. Sorry about that.

It works for me (as long as you put parens around VM_WRITE|VM_SHARED).

Please consider committing it. :-)

Signed-off-by: Suleiman Souhlal <suleiman@google.com>

-- Suleiman
-
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