VM: skip the stack guard page lookup in get_user_pages only for mlock

The logic in __get_user_pages() used to skip the stack guard page lookup whenever the caller wasn't interested in seeing what the actual page was. But Michel Lespinasse points out that there are cases where we don't care about the physical page itself (so 'pages' may be NULL), but do want to make sure a page is mapped into the virtual address space. So using the existence of the "pages" array as an indication of whether to look up the guard page or not isn't actually so great, and we really should just use the FOLL_MLOCK bit. But because that bit was only set for the VM_LOCKED case (and not all vma's necessarily have it, even for mlock()), we couldn't do that originally. Fix that by moving the VM_LOCKED check deeper into the call-chain, which actually simplifies many things. Now mlock() gets simpler, and we can also check for FOLL_MLOCK in __get_user_pages() and the code ends up much more straightforward. Reported-and-reviewed-by: Michel Lespinasse <[email protected]> Cc: [email protected] Signed-off-by: Linus Torvalds <[email protected]>
mna · May 5, 2011 · a1fde08 · a1fde08
1 parent 5895198
commit a1fde08
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Showing 2 changed files with 4 additions and 8 deletions.
diff --git a/mm/memory.c b/mm/memory.c
@@ -1359,7 +1359,7 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
 		 */
 		mark_page_accessed(page);
 	}
-	if (flags & FOLL_MLOCK) {
+	if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
 		/*
 		 * The preliminary mapping check is mainly to avoid the
 		 * pointless overhead of lock_page on the ZERO_PAGE
@@ -1552,10 +1552,9 @@ int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 		}
 
 		/*
-		 * If we don't actually want the page itself,
-		 * and it's the stack guard page, just skip it.
+		 * For mlock, just skip the stack guard page.
 		 */
-		if (!pages && stack_guard_page(vma, start))
+		if ((gup_flags & FOLL_MLOCK) && stack_guard_page(vma, start))
 			goto next_page;
 
 		do {

diff --git a/mm/mlock.c b/mm/mlock.c
@@ -162,7 +162,7 @@ static long __mlock_vma_pages_range(struct vm_area_struct *vma,
 	VM_BUG_ON(end   > vma->vm_end);
 	VM_BUG_ON(!rwsem_is_locked(&mm->mmap_sem));
 
-	gup_flags = FOLL_TOUCH;
+	gup_flags = FOLL_TOUCH | FOLL_MLOCK;
 	/*
 	 * We want to touch writable mappings with a write fault in order
 	 * to break COW, except for shared mappings because these don't COW
@@ -178,9 +178,6 @@ static long __mlock_vma_pages_range(struct vm_area_struct *vma,
 	if (vma->vm_flags & (VM_READ | VM_WRITE | VM_EXEC))
 		gup_flags |= FOLL_FORCE;
 
-	if (vma->vm_flags & VM_LOCKED)
-		gup_flags |= FOLL_MLOCK;
-
 	return __get_user_pages(current, mm, addr, nr_pages, gup_flags,
 				NULL, NULL, nonblocking);
 }