Re: [PATCH v3] mm: prevent endless growth of anon_vma hierarchy

[Daniel Forrest <dan.forrest@ssec.wisc.edu>]

On Wed, Nov 26, 2014 at 10:11:45PM +0400, Konstantin Khlebnikov wrote:

> Constantly forking task causes unlimited grow of anon_vma chain.
> Each next child allocate new level of anon_vmas and links vmas to all
> previous levels because it inherits pages from them. None of anon_vmas
> cannot be freed because there might be pages which points to them.
> 
> This patch adds heuristic which decides to reuse existing anon_vma instead
> of forking new one. It counts vmas and direct descendants for each anon_vma.
> Anon_vma with degree lower than two will be reused at next fork.
> 
> As a result each anon_vma has either alive vma or at least two descendants,
> endless chains are no longer possible and count of anon_vmas is no more than
> two times more than count of vmas.

While I was working on the previous fix for this bug, Andrew Morton
noticed that the error return from anon_vma_clone() was being dropped
and replaced with -ENOMEM (which is not itself a bug because the only
error return value from anon_vma_clone() is -ENOMEM).

I did an audit of callers of anon_vma_clone() and discovered an actual
bug where the error return was being lost.  In __split_vma(), between
Linux 3.11 and 3.12 the code was changed so the err variable is used
before the call to anon_vma_clone() and the default initial value of
-ENOMEM is overwritten.  So a failure of anon_vma_clone() will return
success since err at this point is now zero.

Below is a patch which fixes this bug and also propagates the error
return value from anon_vma_clone() in all cases.

I can send this as a separate patch, but maybe it would be easier if
you were to incorporate it into yours?

Signed-off-by: Daniel Forrest <dan.forrest@ssec.wisc.edu>

---
 mmap.c |   10 +++++++---
 rmap.c |    6 ++++--
 2 files changed, 11 insertions(+), 5 deletions(-)

diff -rup a/mm/mmap.c b/mm/mmap.c
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -776,8 +776,11 @@ again:			remove_next = 1 + (end > next->
 		 * shrinking vma had, to cover any anon pages imported.
 		 */
 		if (exporter && exporter->anon_vma && !importer->anon_vma) {
-			if (anon_vma_clone(importer, exporter))
-				return -ENOMEM;
+			int error;
+
+			error = anon_vma_clone(importer, exporter);
+			if (error)
+				return error;
 			importer->anon_vma = exporter->anon_vma;
 		}
 	}
@@ -2469,7 +2472,8 @@ static int __split_vma(struct mm_struct 
 	if (err)
 		goto out_free_vma;
 
-	if (anon_vma_clone(new, vma))
+	err = anon_vma_clone(new, vma);
+	if (err)
 		goto out_free_mpol;
 
 	if (new->vm_file)
diff -rup a/mm/rmap.c b/mm/rmap.c
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -274,6 +274,7 @@ int anon_vma_fork(struct vm_area_struct 
 {
 	struct anon_vma_chain *avc;
 	struct anon_vma *anon_vma;
+	int error;
 
 	/* Don't bother if the parent process has no anon_vma here. */
 	if (!pvma->anon_vma)
@@ -283,8 +284,9 @@ int anon_vma_fork(struct vm_area_struct 
 	 * First, attach the new VMA to the parent VMA's anon_vmas,
 	 * so rmap can find non-COWed pages in child processes.
 	 */
-	if (anon_vma_clone(vma, pvma))
-		return -ENOMEM;
+	error = anon_vma_clone(vma, pvma);
+	if (error)
+		return error;
 
 	/* Then add our own anon_vma. */
 	anon_vma = anon_vma_alloc();

-- 
Daniel K. Forrest		Space Science and
dan.forrest@ssec.wisc.edu	Engineering Center
(608) 890 - 0558		University of Wisconsin, Madison

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