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

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

[Dan Carpenter]

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>
---
I'm not very familiar with this code, so handle with care.

diff --git a/mm/ksm.c b/mm/ksm.c
index 8cdfc2a..956880f 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -365,7 +365,7 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
 	do {
 		cond_resched();
 		page = follow_page(vma, addr, FOLL_GET);
-		if (!page)
+		if (IS_ERR_OR_NULL(page))
 			break;
 		if (PageKsm(page))
 			ret = handle_mm_fault(vma->vm_mm, vma, addr,
@@ -447,7 +447,7 @@ static struct page *get_mergeable_page(struct rmap_item *rmap_item)
 		goto out;
 
 	page = follow_page(vma, addr, FOLL_GET);
-	if (!page)
+	if (IS_ERR_OR_NULL(page))
 		goto out;
 	if (PageAnon(page)) {
 		flush_anon_page(vma, page, addr);
@@ -1086,7 +1086,7 @@ struct rmap_item *unstable_tree_search_insert(struct rmap_item *rmap_item,
 		cond_resched();
 		tree_rmap_item = rb_entry(*new, struct rmap_item, node);
 		tree_page = get_mergeable_page(tree_rmap_item);
-		if (!tree_page)
+		if (IS_ERR_OR_NULL(tree_page))
 			return NULL;
 
 		/*
@@ -1294,7 +1294,7 @@ next_mm:
 			if (ksm_test_exit(mm))
 				break;
 			*page = follow_page(vma, ksm_scan.address, FOLL_GET);
-			if (*page && PageAnon(*page)) {
+			if (!IS_ERR_OR_NULL(*page) && PageAnon(*page)) {
 				flush_anon_page(vma, *page, ksm_scan.address);
 				flush_dcache_page(*page);
 				rmap_item = get_next_rmap_item(slot,
@@ -1308,7 +1308,7 @@ next_mm:
 				up_read(&mm->mmap_sem);
 				return rmap_item;
 			}
-			if (*page)
+			if (!IS_ERR_OR_NULL(*page))
 				put_page(*page);
 			ksm_scan.address += PAGE_SIZE;
 			cond_resched();
@@ -1367,7 +1367,7 @@ next_mm:
 static void ksm_do_scan(unsigned int scan_npages)
 {
 	struct rmap_item *rmap_item;
-	struct page *page;
+	struct page *uninitialized_var(page);
 
 	while (scan_npages--) {
 		cond_resched();

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

[Rik van Riel]

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>

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

[Andrea Arcangeli]

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.

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

[Johannes Weiner]

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

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

[Andrea Arcangeli]

On Wed, Apr 21, 2010 at 10:53:05PM +0200, Johannes Weiner wrote:
> Check below that loop.  If it returns non-null, the first check is
> whether it IS_ERR().

Indeed.

> + * 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()).

where exactly in vm_normal_page? Note I already checked vm_normal_page
before sending the prev email and I didn't immediately see.  I search
return and they all return NULL except the return pfn_to_page(pfn), so
is pfn_to_page that returns -EFAULT (the implementations I checked
don't but there are plenty that I didn't check...).

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

[Johannes Weiner]

On Wed, Apr 21, 2010 at 10:58:07PM +0200, Andrea Arcangeli wrote:
> On Wed, Apr 21, 2010 at 10:53:05PM +0200, Johannes Weiner wrote:
> > + * 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()).
> 
> where exactly in vm_normal_page? Note I already checked vm_normal_page
> before sending the prev email and I didn't immediately see.  I search
> return and they all return NULL except the return pfn_to_page(pfn), so
> is pfn_to_page that returns -EFAULT (the implementations I checked
> don't but there are plenty that I didn't check...).

It's not vm_normal_page() that returns -EFAULT.  It is follow_page()
that translates NULL from vm_normal_page() into -EFAULT.

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

[Izik Eidus]

On Wed, 21 Apr 2010 12:27:59 +0200
Dan Carpenter <error27@gmail.com> wrote:

Hello

> 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>
> ---
> I'm not very familiar with this code, so handle with care.


Acked-by: Izik Eidus <ieidus@redhat.com>

Thanks.

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