Re: [patch] sg: simplify page_count manipulations

[Hugh Dickins <hugh@veritas.com>]

On Fri, 20 Jan 2006, Andrew Morton wrote:
> Nick Piggin <npiggin@suse.de> wrote:
> > On Thu, Jan 19, 2006 at 02:05:25PM -0800, Andrew Morton wrote:
> > > Nick Piggin <npiggin@suse.de> wrote:
> > > >
> > > > On Wed, Jan 18, 2006 at 07:59:37PM -0800, Andrew Morton wrote:
> > > > > Nick Piggin <npiggin@suse.de> wrote:
> > > > > > -	/* N.B. correction _not_ applied to base page of each allocation */
> > > > > > -	for (k = 0; k < rsv_schp->k_use_sg; ++k, ++sg) {
> > > > > > -		for (m = PAGE_SIZE; m < sg->length; m += PAGE_SIZE) {
> > > > > > -			page = sg->page;
> > > > > > -			if (startFinish)
> > > > > > -				get_page(page);
> > > > > > -			else {
> > > > > > -				if (page_count(page) > 0)
> > > > > > -					__put_page(page);
> > > > > > -			}
> > > > > > -		}
> > > > > > -	}
> > > > > > -}
> > > > > 
> > > > > What on earth is the above trying to do?  The inner loop is a rather
> > > > > complex way of doing atomic_add(&page->count, sg->length/PAGE_SIZE).  One
> > > > > suspects there's a missing "[m]" in there.
> > > > > 
> > > > 
> > > > It does this on the first mmap of the device, in the hope that subsequent
> > > > nopage, unmaps would not free the constituent pages in the scatterlist.
> > > > 
> > > 
> > > But it's doing it wrongly, isn't it?  Or am I completely nuts?
> > 
> > No I think you're right. I'm not sure why this doesn't oops but I
> > thought it was the (main) reason others wanted to get rid of this
> > convoluted code earlier on. I see nobody else is planning to do anything
> > about it though, so I figure I must have missed the reason why it isn't
> > a problem.
> > 
> > But either way I don't think the code actually _does_ anything, even if
> > its bugginess doesn't actually lead to a bug.
> > 
> 
> I suspect nobody tried to munmap pages beyond the first one.
> 
> Yes, let's use a compound page in there and I expect Doug will be able to
> test it for us sometime.

That function did move page along in 2.6.15, but has got screwed up since:
good reason, I think, to speed Nick's patch through to clean it all away.

Hugh