Re: [patch 2.6.13-rc4] fix get_user_pages bug

[Linus Torvalds]

On Wed, 3 Aug 2005, Nick Piggin wrote:
> 
> Oh, it gets rid of the -1 for VM_FAULT_OOM. Doesn't seem like there
> is a good reason for it, but might that break out of tree drivers?

Ok, I applied this because it was reasonably pretty and I liked the 
approach. It seems buggy, though, since it was using "switch ()" to test 
the bits (wrongly, afaik), and I'm going to apply the appended on top of 
it. Holler quickly if you disagreee..

		Linus
----
diff --git a/mm/memory.c b/mm/memory.c
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -949,6 +949,8 @@ int get_user_pages(struct task_struct *t
 
 			cond_resched_lock(&mm->page_table_lock);
 			while (!(page = follow_page(mm, start, write_access))) {
+				int ret;
+
 				/*
 				 * Shortcut for anonymous pages. We don't want
 				 * to force the creation of pages tables for
@@ -961,16 +963,18 @@ int get_user_pages(struct task_struct *t
 					break;
 				}
 				spin_unlock(&mm->page_table_lock);
-				switch (__handle_mm_fault(mm, vma, start,
-							write_access)) {
-				case VM_FAULT_WRITE:
-					/*
-					 * do_wp_page has broken COW when
-					 * necessary, even if maybe_mkwrite
-					 * decided not to set pte_write
-					 */
+				ret = __handle_mm_fault(mm, vma, start, write_access);
+
+				/*
+				 * The VM_FAULT_WRITE bit tells us that do_wp_page has
+				 * broken COW when necessary, even if maybe_mkwrite
+				 * decided not to set pte_write. We can thus safely do
+				 * subsequent page lookups as if they were reads.
+				 */
+				if (ret & VM_FAULT_WRITE)
 					write_access = 0;
-					/* FALLTHRU */
+				
+				switch (ret & ~VM_FAULT_WRITE) {
 				case VM_FAULT_MINOR:
 					tsk->min_flt++;
 					break;
-
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