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Subject[RFC PATCH] Re: Repeated fork() causes SLAB to grow without bound
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 <walken@google.com>
---
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 > 1)
+ 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;
--
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.


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