Re: [patch] ksm: check for ERR_PTR from follow_page()

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Apr 21, 2010 at 07:46:15PM +0200, Andrea Arcangeli wrote:
> On Wed, Apr 21, 2010 at 11:24:24AM -0400, Rik van Riel wrote:
> > On 04/21/2010 06:27 AM, Dan Carpenter wrote:
> > > The follow_page() function can potentially return -EFAULT so I added
> > > checks for this.
> > >
> > > Also I silenced an uninitialized variable warning on my version of gcc
> > > (version 4.3.2).
> > >
> > > Signed-off-by: Dan Carpenter<error27@gmail.com>
> > 
> > Acked-by: Rik van Riel <riel@redhat.com>
> 
>   	    	while (!(page = follow_page(vma, start, foll_flags)))
>   	    	{
> 
> gup only checks for null, so when exactly is follow_page going to
> return -EFAULT? It's not immediately clear.

Check below that loop.  If it returns non-null, the first check is
whether it IS_ERR().

How about the below?

	Hannes

---
From: Johannes Weiner <hannes@cmpxchg.org>
Subject: mm: document follow_page()

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/memory.c |   13 +++++++++++--
 1 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 833952d..119b7cc 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1227,8 +1227,17 @@ int zap_vma_ptes(struct vm_area_struct *vma, unsigned long address,
 }
 EXPORT_SYMBOL_GPL(zap_vma_ptes);
 
-/*
- * Do a quick page-table lookup for a single page.
+/**
+ * follow_page - look up a page descriptor from a user-virtual address
+ * @vma: vm_area_struct mapping @address
+ * @address: virtual address to look up
+ * @flags: flags modifying lookup behaviour
+ *
+ * @flags can have FOLL_ flags set, defined in <linux/mm.h>
+ *
+ * Returns the mapped (struct page *), %NULL if no mapping exists, or
+ * an error pointer if there is a mapping to something not represented
+ * by a page descriptor (see also vm_normal_page()).
  */
 struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
 			unsigned int flags)
-- 
1.7.0.2