On Tue, Aug 21, 2012 at 11:29:54PM -0400, Rik van Riel wrote:
> On 08/21/2012 11:20 PM, Michel Lespinasse wrote:
> >On Mon, Aug 20, 2012 at 02:39:26AM -0700, Michel Lespinasse wrote:
> >>Instead of adding an atomic count for page references, we could limit
> >>the anon_vma stacking depth. In fork, we would only clone anon_vmas
> >>that have a low enough generation count. I think that's not great
> >>(adds a special case for the deep-fork-without-exec behavior), but
> >>still better than the atomic page reference counter.
> >
> >Here is an attached patch to demonstrate the idea.
> >
> >anon_vma_clone() is modified to return the length of the existing same_vma
> >anon vma chain, and we create a new anon_vma in the child only on the first
> >fork (this could be tweaked to allow up to a set number of forks, but
> >I think the first fork would cover all the common forking server cases).
>
> I suspect we need 2 or 3.
>
> Some forking servers first fork off one child, and have
> the original parent exit, in order to "background the server".
> That first child then becomes the parent to the real child
> processes that do the work.
>
> It is conceivable that we might need an extra level for
> processes that do something special with privilege dropping,
> namespace changing, etc...
>
> Even setting the threshold to 5 should be totally harmless,
> since the problem does not kick in until we have really
> long chains, like in Dan's bug report.
I have been running with Michel's patch (with the threshold set to 5)
for quite a few months now and can confirm that it does indeed solve
my problem. I am not a kernel developer, so I would appreciate if one
of you could push this into the kernel tree.
NOTE: I have attached Michel's patch with "(length > 1)" modified to
"(length > 5)" and added a "Tested-by:".
---
On Mon, Aug 20, 2012 at 02:39:26AM -0700, Michel Lespinasse wrote:
> Instead of adding an atomic count for page references, we could limit
> the anon_vma stacking depth. In fork, we would only clone anon_vmas
> that have a low enough generation count. I think that's not great
> (adds a special case for the deep-fork-without-exec behavior), but
> still better than the atomic page reference counter.
Here is an attached patch to demonstrate the idea.
anon_vma_clone() is modified to return the length of the existing same_vma
anon vma chain, and we create a new anon_vma in the child only on the first
fork (this could be tweaked to allow up to a set number of forks, but
I think the first fork would cover all the common forking server cases).
Signed-off-by: Michel Lespinasse <wal...@google.com>
Tested-by: Daniel Forrest <dan.forr...@ssec.wisc.edu>
---
mm/mmap.c | 6 +++---
mm/rmap.c | 18 ++++++++++++++----
2 files changed, 17 insertions(+), 7 deletions(-)
diff --git a/mm/mmap.c b/mm/mmap.c
index 3edfcdfa42d9..e14b19a838cb 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -539,7 +539,7 @@ again: remove_next = 1 + (end >
next->vm_end);
* shrinking vma had, to cover any anon pages imported.
*/
if (exporter && exporter->anon_vma && !importer->anon_vma) {
- if (anon_vma_clone(importer, exporter))
+ if (anon_vma_clone(importer, exporter) < 0)
return -ENOMEM;
importer->anon_vma = exporter->anon_vma;
}
@@ -1988,7 +1988,7 @@ static int __split_vma(struct mm_struct * mm, struct
vm_area_struct * vma,
}
vma_set_policy(new, pol);
- if (anon_vma_clone(new, vma))
+ if (anon_vma_clone(new, vma) < 0)
goto out_free_mpol;
if (new->vm_file) {
@@ -2409,7 +2409,7 @@ struct vm_area_struct *copy_vma(struct vm_area_struct
**vmap,
if (IS_ERR(pol))
goto out_free_vma;
INIT_LIST_HEAD(&new_vma->anon_vma_chain);
- if (anon_vma_clone(new_vma, vma))
+ if (anon_vma_clone(new_vma, vma) < 0)
goto out_free_mempol;
vma_set_policy(new_vma, pol);
new_vma->vm_start = addr;
diff --git a/mm/rmap.c b/mm/rmap.c
index 0f3b7cda2a24..ba8a726aaee6 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -238,12 +238,13 @@ static inline void unlock_anon_vma_root(struct anon_vma
*root)
/*
* Attach the anon_vmas from src to dst.
- * Returns 0 on success, -ENOMEM on failure.
+ * Returns length of the anon_vma chain on success, -ENOMEM on failure.
*/
int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
{
struct anon_vma_chain *avc, *pavc;
struct anon_vma *root = NULL;
+ int length = 0;
list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
struct anon_vma *anon_vma;
@@ -259,9 +260,10 @@ int anon_vma_clone(struct vm_area_struct *dst, struct
vm_area_struct *src)
anon_vma = pavc->anon_vma;
root = lock_anon_vma_root(root, anon_vma);
anon_vma_chain_link(dst, avc, anon_vma);
+ length++;
}
unlock_anon_vma_root(root);
- return 0;
+ return length;
enomem_failure:
unlink_anon_vmas(dst);
@@ -322,6 +324,7 @@ int anon_vma_fork(struct vm_area_struct *vma, struct
vm_area_struct *pvma)
{
struct anon_vma_chain *avc;
struct anon_vma *anon_vma;
+ int length;
/* Don't bother if the parent process has no anon_vma here. */
if (!pvma->anon_vma)
@@ -331,10 +334,17 @@ int anon_vma_fork(struct vm_area_struct *vma, struct
vm_area_struct *pvma)
* 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))
+ length = anon_vma_clone(vma, pvma);
+ if (length < 0)
return -ENOMEM;
+ else if (length > 5)
+ return 0;
- /* Then add our own anon_vma. */
+ /*
+ * Then add our own anon_vma. We do this only on the first fork after
+ * the anon_vma is created, as we don't want the same_vma chain to
+ * grow arbitrarily large.
+ */
anon_vma = anon_vma_alloc();
if (!anon_vma)
goto out_error;
--
Daniel K. Forrest Space Science and
dan.forr...@ssec.wisc.edu Engineering Center
(608) 890 - 0558 University of Wisconsin, Madison
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