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Date: Mon, 13 Aug 2012 19:17:01 -0700 (PDT)
From: Hugh Dickins <hughd@...gle.com>
To: Hillf Danton <dhillf@...il.com>
cc: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
Mikulas Patocka <mpatocka@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Benny Halevy <bhalevy@...ian.com>,
LKML <linux-kernel@...r.kernel.org>,
Linux-MM <linux-mm@...ck.org>
Subject: Re: [patch] mmap: feed back correct prev vma when finding vma
On Fri, 10 Aug 2012, Hillf Danton wrote:
> On Fri, Aug 10, 2012 at 9:26 AM, Hugh Dickins <hughd@...gle.com> wrote:
> > On Thu, 9 Aug 2012, Hillf Danton wrote:
> >> After walking rb tree, if vma is determined, prev vma has to be determined
> >> based on vma; and rb_prev should be considered only if no vma determined.
> >
> > Why? Because you think more code is better code? I disagree.
>
> s/more/correct/
>
> Because feedback is incorrect if we return vma corresponding to
> the root node.
"feedback"? "root node"?
I took another look, and a WARN_ON soon told me that you do have a point.
It stirred memories, and I found an earlier thread from 2008, responsible
for replacing the original "return vma" by a "break" in 2.6.27.
I agree that find_vma_prepare() is confusing, but your vm_prev patch is not
the right fix: let's un-confuse it this way, which barely needs comment.
Hugh
[PATCH] mm: replace find_vma_prepare by clearer find_vma_links
People get confused by find_vma_prepare(), because it doesn't care about
what it returns in its output args, when its callers won't be interested.
Clarify by passing in end-of-range address too, and returning failure if
any existing vma overlaps the new range: instead of returning an ambiguous
vma which most callers then must check. find_vma_links() is a clearer name.
This does revert 2.6.27's dfe195fb79e88 ("mm: fix uninitialized variables
for find_vma_prepare callers"), but it looks like gcc 4.3.0 was one of
those releases too eager to shout about uninitialized variables: only
copy_vma() warns with 4.5.1 and 4.7.1, which a BUG on error silences.
Signed-off-by: Hugh Dickins <hughd@...gle.com>
Cc: Benny Halevy <bhalevy@...ian.com>
Cc: Hillf Danton <dhillf@...il.com>
---
mm/mmap.c | 45 +++++++++++++++++++++------------------------
1 file changed, 21 insertions(+), 24 deletions(-)
--- 3.6-rc1/mm/mmap.c 2012-08-03 08:31:27.064842271 -0700
+++ linux/mm/mmap.c 2012-08-13 12:23:35.862895633 -0700
@@ -356,17 +356,14 @@ void validate_mm(struct mm_struct *mm)
#define validate_mm(mm) do { } while (0)
#endif
-static struct vm_area_struct *
-find_vma_prepare(struct mm_struct *mm, unsigned long addr,
- struct vm_area_struct **pprev, struct rb_node ***rb_link,
- struct rb_node ** rb_parent)
+static int find_vma_links(struct mm_struct *mm, unsigned long addr,
+ unsigned long end, struct vm_area_struct **pprev,
+ struct rb_node ***rb_link, struct rb_node **rb_parent)
{
- struct vm_area_struct * vma;
- struct rb_node ** __rb_link, * __rb_parent, * rb_prev;
+ struct rb_node **__rb_link, *__rb_parent, *rb_prev;
__rb_link = &mm->mm_rb.rb_node;
rb_prev = __rb_parent = NULL;
- vma = NULL;
while (*__rb_link) {
struct vm_area_struct *vma_tmp;
@@ -375,9 +372,9 @@ find_vma_prepare(struct mm_struct *mm, u
vma_tmp = rb_entry(__rb_parent, struct vm_area_struct, vm_rb);
if (vma_tmp->vm_end > addr) {
- vma = vma_tmp;
- if (vma_tmp->vm_start <= addr)
- break;
+ /* Fail if an existing vma overlaps the area */
+ if (vma_tmp->vm_start < end)
+ return -ENOMEM;
__rb_link = &__rb_parent->rb_left;
} else {
rb_prev = __rb_parent;
@@ -390,7 +387,7 @@ find_vma_prepare(struct mm_struct *mm, u
*pprev = rb_entry(rb_prev, struct vm_area_struct, vm_rb);
*rb_link = __rb_link;
*rb_parent = __rb_parent;
- return vma;
+ return 0;
}
void __vma_link_rb(struct mm_struct *mm, struct vm_area_struct *vma,
@@ -459,11 +456,12 @@ static void vma_link(struct mm_struct *m
*/
static void __insert_vm_struct(struct mm_struct *mm, struct vm_area_struct *vma)
{
- struct vm_area_struct *__vma, *prev;
+ struct vm_area_struct *prev;
struct rb_node **rb_link, *rb_parent;
- __vma = find_vma_prepare(mm, vma->vm_start,&prev, &rb_link, &rb_parent);
- BUG_ON(__vma && __vma->vm_start < vma->vm_end);
+ if (find_vma_links(mm, vma->vm_start, vma->vm_end,
+ &prev, &rb_link, &rb_parent))
+ BUG();
__vma_link(mm, vma, prev, rb_link, rb_parent);
mm->map_count++;
}
@@ -1229,8 +1227,7 @@ unsigned long mmap_region(struct file *f
/* Clear old maps */
error = -ENOMEM;
munmap_back:
- vma = find_vma_prepare(mm, addr, &prev, &rb_link, &rb_parent);
- if (vma && vma->vm_start < addr + len) {
+ if (find_vma_links(mm, addr, addr + len, &prev, &rb_link, &rb_parent)) {
if (do_munmap(mm, addr, len))
return -ENOMEM;
goto munmap_back;
@@ -2201,8 +2198,7 @@ static unsigned long do_brk(unsigned lon
* Clear old maps. this also does some error checking for us
*/
munmap_back:
- vma = find_vma_prepare(mm, addr, &prev, &rb_link, &rb_parent);
- if (vma && vma->vm_start < addr + len) {
+ if (find_vma_links(mm, addr, addr + len, &prev, &rb_link, &rb_parent)) {
if (do_munmap(mm, addr, len))
return -ENOMEM;
goto munmap_back;
@@ -2316,10 +2312,10 @@ void exit_mmap(struct mm_struct *mm)
* and into the inode's i_mmap tree. If vm_file is non-NULL
* then i_mmap_mutex is taken here.
*/
-int insert_vm_struct(struct mm_struct * mm, struct vm_area_struct * vma)
+int insert_vm_struct(struct mm_struct *mm, struct vm_area_struct *vma)
{
- struct vm_area_struct * __vma, * prev;
- struct rb_node ** rb_link, * rb_parent;
+ struct vm_area_struct *prev;
+ struct rb_node **rb_link, *rb_parent;
/*
* The vm_pgoff of a purely anonymous vma should be irrelevant
@@ -2337,8 +2333,8 @@ int insert_vm_struct(struct mm_struct *
BUG_ON(vma->anon_vma);
vma->vm_pgoff = vma->vm_start >> PAGE_SHIFT;
}
- __vma = find_vma_prepare(mm,vma->vm_start,&prev,&rb_link,&rb_parent);
- if (__vma && __vma->vm_start < vma->vm_end)
+ if (find_vma_links(mm, vma->vm_start, vma->vm_end,
+ &prev, &rb_link, &rb_parent))
return -ENOMEM;
if ((vma->vm_flags & VM_ACCOUNT) &&
security_vm_enough_memory_mm(mm, vma_pages(vma)))
@@ -2372,7 +2368,8 @@ struct vm_area_struct *copy_vma(struct v
faulted_in_anon_vma = false;
}
- find_vma_prepare(mm, addr, &prev, &rb_link, &rb_parent);
+ if (find_vma_links(mm, addr, addr + len, &prev, &rb_link, &rb_parent))
+ BUG();
new_vma = vma_merge(mm, prev, addr, addr + len, vma->vm_flags,
vma->anon_vma, vma->vm_file, pgoff, vma_policy(vma));
if (new_vma) {
--
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