Re: page fault scalability patch V16 [3/4]: Drop page_table_lock inhandle_mm_fault

[Christoph Lameter]

On Wed, 2 Feb 2005, Nick Piggin wrote:

> Well yeah, but the interesting case is when that isn't a lock ;)
>
> I'm not saying what you've got is no good. I'm sure it would be fine
> for testing. And if it happens that we can do the "page_count doesn't
> mean anything after it has reached zero and been freed. Nor will it
> necessarily be zero when a new page is allocated" thing without many
> problems, then this may be a fine way to do it.
>
> I was just pointing out this could be a problem without putting a
> lot of thought into it...

Surely we need to do this the right way. Do we really need to
use page_cache_get()? Is anything relying on page_count == 2 of
the old_page?

I mean we could just speculatively copy, risk copying crap and
discard that later when we find that the pte has changed. This would
simplify the function:

Index: linux-2.6.10/mm/memory.c
===================================================================
--- linux-2.6.10.orig/mm/memory.c	2005-02-01 18:10:46.000000000 -0800
+++ linux-2.6.10/mm/memory.c	2005-02-01 18:43:08.000000000 -0800
@@ -1323,9 +1323,6 @@ static int do_wp_page(struct mm_struct *
 	/*
 	 * Ok, we need to copy. Oh, well..
 	 */
-	if (!PageReserved(old_page))
-		page_cache_get(old_page);
-
 	if (unlikely(anon_vma_prepare(vma)))
 		goto no_new_page;
 	if (old_page == ZERO_PAGE(address)) {
@@ -1336,6 +1333,10 @@ static int do_wp_page(struct mm_struct *
 		new_page = alloc_page_vma(GFP_HIGHUSER, vma, address);
 		if (!new_page)
 			goto no_new_page;
+		/*
+		 * No page_cache_get so we may copy some crap
+		 * that is later discarded if the pte has changed
+		 */
 		copy_user_highpage(new_page, old_page, address);
 	}
 	/*
@@ -1352,7 +1353,6 @@ static int do_wp_page(struct mm_struct *
 			acct_update_integrals();
 			update_mem_hiwater();
 		} else
-
 			page_remove_rmap(old_page);
 		break_cow(vma, new_page, address, page_table);
 		lru_cache_add_active(new_page);
@@ -1363,7 +1363,6 @@ static int do_wp_page(struct mm_struct *
 	}
 	pte_unmap(page_table);
 	page_cache_release(new_page);
-	page_cache_release(old_page);
 	spin_unlock(&mm->page_table_lock);
 	return VM_FAULT_MINOR;

-
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