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SubjectRe: [PATCH 2/2] mm,migration: Fix race between shift_arg_pages and rmap_walk by guaranteeing rmap_walk finds PTEs created within the temporary stack
On Thu, 6 May 2010 19:12:59 -0700 (PDT)
Linus Torvalds <torvalds@linux-foundation.org> wrote:

>
>
> On Fri, 7 May 2010, KAMEZAWA Hiroyuki wrote:
> >
> > IIUC, move_page_tables() may call "page table allocation" and it cannot be
> > done under spinlock.
>
> Bah. It only does a "alloc_new_pmd()", and we could easily move that out
> of the loop and pre-allocate the pmd's.
>
> If that's the only reason, then it's a really weak one, methinks.
>
Hmm, is this too slow ? This is the simplest one I have.

==
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

exec()'s shift_arg_pages calls adjust_vma and move_page_table() directly.
During this, rmap information (i.e. page <-> pte <-> address <-> vma)
information is inconsistent. This causes a bug in rmap_walk() which
rmap_walk cannot find valid ptes it must visit.

Considering the race, move_vma() does valid things. So, making use of
move_vma() instead of bare move_page_tables() is a choice.

Pros.
- all races are handled under /mm. very simple.
Cons.
- This may makes exec() slow a bit.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
fs/exec.c | 64 +++++++++++++++--------------------------------------
include/linux/mm.h | 6 ++--
mm/mremap.c | 4 +--
3 files changed, 24 insertions(+), 50 deletions(-)
Index: linux-2.6.34-rc5-mm1/fs/exec.c
===================================================================
--- linux-2.6.34-rc5-mm1.orig/fs/exec.c
+++ linux-2.6.34-rc5-mm1/fs/exec.c
@@ -486,14 +486,7 @@ EXPORT_SYMBOL(copy_strings_kernel);
/*
* During bprm_mm_init(), we create a temporary stack at STACK_TOP_MAX. Once
* the binfmt code determines where the new stack should reside, we shift it to
- * its final location. The process proceeds as follows:
- *
- * 1) Use shift to calculate the new vma endpoints.
- * 2) Extend vma to cover both the old and new ranges. This ensures the
- * arguments passed to subsequent functions are consistent.
- * 3) Move vma's page tables to the new range.
- * 4) Free up any cleared pgd range.
- * 5) Shrink the vma to cover only the new range.
+ * its final location.
*/
static int shift_arg_pages(struct vm_area_struct *vma, unsigned long shift)
{
@@ -503,7 +496,7 @@ static int shift_arg_pages(struct vm_are
unsigned long length = old_end - old_start;
unsigned long new_start = old_start - shift;
unsigned long new_end = old_end - shift;
- struct mmu_gather *tlb;
+ unsigned long ret;

BUG_ON(new_start > new_end);

@@ -514,45 +507,23 @@ static int shift_arg_pages(struct vm_are
if (vma != find_vma(mm, new_start))
return -EFAULT;

- /*
- * cover the whole range: [new_start, old_end)
- */
- if (vma_adjust(vma, new_start, old_end, vma->vm_pgoff, NULL))
- return -ENOMEM;
-
- /*
- * move the page tables downwards, on failure we rely on
- * process cleanup to remove whatever mess we made.
- */
- if (length != move_page_tables(vma, old_start,
- vma, new_start, length))
- return -ENOMEM;
+ if (new_end > old_start) { /* overlap */
+ unsigned long part_len = new_end - old_start;
+ ret = move_vma(vma, old_start, part_len, part_len, new_start);

- lru_add_drain();
- tlb = tlb_gather_mmu(mm, 0);
- if (new_end > old_start) {
- /*
- * when the old and new regions overlap clear from new_end.
- */
- free_pgd_range(tlb, new_end, old_end, new_end,
- vma->vm_next ? vma->vm_next->vm_start : 0);
+ if (ret != new_start)
+ return -ENOMEM;
+ /* old_vma is splitted.. */
+ vma = find_vma(mm, old_start + part_len);
+ ret = move_vma(vma, old_start + part_len, length - part_len,
+ length - part_len, new_start + part_len);
+ if (ret != new_start + part_len)
+ return -ENOMEM;
} else {
- /*
- * otherwise, clean from old_start; this is done to not touch
- * the address space in [new_end, old_start) some architectures
- * have constraints on va-space that make this illegal (IA64) -
- * for the others its just a little faster.
- */
- free_pgd_range(tlb, old_start, old_end, new_end,
- vma->vm_next ? vma->vm_next->vm_start : 0);
+ ret = move_vma(vma, old_start, length, length, new_start);
+ if (ret != new_start)
+ return -ENOMEM;
}
- tlb_finish_mmu(tlb, new_end, old_end);
-
- /*
- * Shrink the vma to just the new range. Always succeeds.
- */
- vma_adjust(vma, new_start, new_end, vma->vm_pgoff, NULL);
-
return 0;
}

@@ -625,9 +596,12 @@ int setup_arg_pages(struct linux_binprm

/* Move stack pages down in memory. */
if (stack_shift) {
+ unsigned long new_start = vma->vm_start - stack_shift;
ret = shift_arg_pages(vma, stack_shift);
if (ret)
goto out_unlock;
+ vma = find_vma(mm, new_start);
+ bprm->vma = vma;
}

stack_expand = 131072UL; /* randomly 32*4k (or 2*64k) pages */
Index: linux-2.6.34-rc5-mm1/include/linux/mm.h
===================================================================
--- linux-2.6.34-rc5-mm1.orig/include/linux/mm.h
+++ linux-2.6.34-rc5-mm1/include/linux/mm.h
@@ -856,9 +856,9 @@ int set_page_dirty_lock(struct page *pag
int set_page_dirty_notag(struct page *page);
int clear_page_dirty_for_io(struct page *page);

-extern unsigned long move_page_tables(struct vm_area_struct *vma,
- unsigned long old_addr, struct vm_area_struct *new_vma,
- unsigned long new_addr, unsigned long len);
+extern unsigned long move_vma(struct vm_area_struct *vma,
+ unsigned long old_addr, unsigned long old_length,
+ unsigned long new_length, unsigned long new_addr);
extern unsigned long do_mremap(unsigned long addr,
unsigned long old_len, unsigned long new_len,
unsigned long flags, unsigned long new_addr);
Index: linux-2.6.34-rc5-mm1/mm/mremap.c
===================================================================
--- linux-2.6.34-rc5-mm1.orig/mm/mremap.c
+++ linux-2.6.34-rc5-mm1/mm/mremap.c
@@ -128,7 +128,7 @@ static void move_ptes(struct vm_area_str

#define LATENCY_LIMIT (64 * PAGE_SIZE)

-unsigned long move_page_tables(struct vm_area_struct *vma,
+static unsigned long move_page_tables(struct vm_area_struct *vma,
unsigned long old_addr, struct vm_area_struct *new_vma,
unsigned long new_addr, unsigned long len)
{
@@ -162,7 +162,7 @@ unsigned long move_page_tables(struct vm
return len + old_addr - old_end; /* how much done */
}

-static unsigned long move_vma(struct vm_area_struct *vma,
+unsigned long move_vma(struct vm_area_struct *vma,
unsigned long old_addr, unsigned long old_len,
unsigned long new_len, unsigned long new_addr)
{


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