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@xxxxxxxxxxxxx>
---
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@xxxxxxxxxxxxx Engineering Center
(608) 890 - 0558 University of Wisconsin, Madison
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>