Re: [RFC][PATCH 1/3] radix priority search tree - objrmap complexity fix

Andrea Arcangeli
2004-04-03T23:27:17Z

On Sat, Apr 03, 2004 at 12:02:27PM -0800, Andrew Morton wrote:
> It might be better to switch over to address masking in get_user_pages()
> and just dump all the compound page logic. I don't immediately see how the

I'm all for it, this is how the 2.4 get_user_pages deals with bigpages
too, I've never enjoyed the compound thing.

> get_user_pages() caller can subsequently do put_page() against the correct
> pageframe, but I assume you worked that out?

see this patch:

http://www.us.kernel.org/pub/linux/kernel/people/andrea/kernels/v2.4/2.4.23aa2/9910_shm-largepage-18.gz

it's a two liner fix in follow_page:

@@ -439,6 +457,8 @@ static struct page * follow_page(struct
pmd = pmd_offset(pgd, address);
if (pmd_none(*pmd))
goto out;
+ if (pmd_bigpage(*pmd))
+ return __pmd_page(*pmd) + (address & BIGPAGE_MASK) / PAGE_SIZE;

the BIGPAGE_MASK will never expose anything but the page->private to the
get_user_pages code, and handle_mm_fault takes care of doing the page
faults properly using larepages and pmds if the vma is marked
VM_BIGPAGE.

rawio on largepages has been a must-have feature (especially on >=32G)
for more than one year, definitely no need of compound slowdown for that.

Still I would like to understand what's wrong in Christoph's ppc machine
before dumping the whole compound thing.