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Date: Wed, 26 Nov 2014 22:11:45 +0400
From: Konstantin Khlebnikov <koct9i@...il.com>
To: linux-mm <linux-mm@...ck.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc: Andrea Arcangeli <aarcange@...hat.com>,
Rik van Riel <riel@...hat.com>,
Tim Hartrick <tim@...ecast.com>,
Daniel Forrest <dan.forrest@...c.wisc.edu>,
Hugh Dickins <hughd@...gle.com>,
Michel Lespinasse <walken@...gle.com>,
Vlastimil Babka <vbabka@...e.cz>
Subject: [PATCH v3] mm: prevent endless growth of anon_vma hierarchy
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.
Signed-off-by: Konstantin Khlebnikov <koct9i@...il.com>
Reported-by: Daniel Forrest <dan.forrest@...c.wisc.edu>
Tested-by: Michal Hocko <mhocko@...e.cz>
Reviewed-by: Michal Hocko <mhocko@...e.cz>
Link: http://lkml.kernel.org/r/20120816024610.GA5350@evergreen.ssec.wisc.edu
Fixes: 5beb49305251 ("mm: change anon_vma linking to fix multi-process server scalability issue")
Cc: Stable <stable@...r.kernel.org> (2.6.34+)
---
v2: update degree in anon_vma_prepare for merged anon_vma
v3: update comment and tags
---
include/linux/rmap.h | 16 ++++++++++++++++
mm/rmap.c | 30 +++++++++++++++++++++++++++++-
2 files changed, 45 insertions(+), 1 deletion(-)
diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index c0c2bce..b1d140c 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -45,6 +45,22 @@ struct anon_vma {
* mm_take_all_locks() (mm_all_locks_mutex).
*/
struct rb_root rb_root; /* Interval tree of private "related" vmas */
+
+ /*
+ * Count of child anon_vmas and VMAs which points to this anon_vma.
+ *
+ * This counter is used for making decision about reusing old anon_vma
+ * instead of forking new one. It allows to detect anon_vmas which have
+ * just one direct descendant and no vmas. Reusing such anon_vma not
+ * leads to significant preformance regression but prevents degradation
+ * of anon_vma hierarchy to endless linear chain.
+ *
+ * Root anon_vma is never reused because it is its own parent and it has
+ * at leat one vma or child, thus at fork it's degree is at least 2.
+ */
+ unsigned degree;
+
+ struct anon_vma *parent; /* Parent of this anon_vma */
};
/*
diff --git a/mm/rmap.c b/mm/rmap.c
index 19886fb..df5c44e 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -72,6 +72,8 @@ static inline struct anon_vma *anon_vma_alloc(void)
anon_vma = kmem_cache_alloc(anon_vma_cachep, GFP_KERNEL);
if (anon_vma) {
atomic_set(&anon_vma->refcount, 1);
+ anon_vma->degree = 1; /* Reference for first vma */
+ anon_vma->parent = anon_vma;
/*
* Initialise the anon_vma root to point to itself. If called
* from fork, the root will be reset to the parents anon_vma.
@@ -188,6 +190,8 @@ int anon_vma_prepare(struct vm_area_struct *vma)
if (likely(!vma->anon_vma)) {
vma->anon_vma = anon_vma;
anon_vma_chain_link(vma, avc, anon_vma);
+ /* vma link if merged or child link for new root */
+ anon_vma->degree++;
allocated = NULL;
avc = NULL;
}
@@ -256,7 +260,17 @@ 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);
+
+ /*
+ * Reuse existing anon_vma if its degree lower than two,
+ * that means it has no vma and just one anon_vma child.
+ */
+ if (!dst->anon_vma && anon_vma != src->anon_vma &&
+ anon_vma->degree < 2)
+ dst->anon_vma = anon_vma;
}
+ if (dst->anon_vma)
+ dst->anon_vma->degree++;
unlock_anon_vma_root(root);
return 0;
@@ -279,6 +293,9 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
if (!pvma->anon_vma)
return 0;
+ /* Drop inherited anon_vma, we'll reuse old one or allocate new. */
+ vma->anon_vma = NULL;
+
/*
* First, attach the new VMA to the parent VMA's anon_vmas,
* so rmap can find non-COWed pages in child processes.
@@ -286,6 +303,10 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
if (anon_vma_clone(vma, pvma))
return -ENOMEM;
+ /* An old anon_vma has been reused. */
+ if (vma->anon_vma)
+ return 0;
+
/* Then add our own anon_vma. */
anon_vma = anon_vma_alloc();
if (!anon_vma)
@@ -299,6 +320,7 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
* lock any of the anon_vmas in this anon_vma tree.
*/
anon_vma->root = pvma->anon_vma->root;
+ anon_vma->parent = pvma->anon_vma;
/*
* With refcounts, an anon_vma can stay around longer than the
* process it belongs to. The root anon_vma needs to be pinned until
@@ -309,6 +331,7 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
vma->anon_vma = anon_vma;
anon_vma_lock_write(anon_vma);
anon_vma_chain_link(vma, avc, anon_vma);
+ anon_vma->parent->degree++;
anon_vma_unlock_write(anon_vma);
return 0;
@@ -339,12 +362,16 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
* Leave empty anon_vmas on the list - we'll need
* to free them outside the lock.
*/
- if (RB_EMPTY_ROOT(&anon_vma->rb_root))
+ if (RB_EMPTY_ROOT(&anon_vma->rb_root)) {
+ anon_vma->parent->degree--;
continue;
+ }
list_del(&avc->same_vma);
anon_vma_chain_free(avc);
}
+ if (vma->anon_vma)
+ vma->anon_vma->degree--;
unlock_anon_vma_root(root);
/*
@@ -355,6 +382,7 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
struct anon_vma *anon_vma = avc->anon_vma;
+ BUG_ON(anon_vma->degree);
put_anon_vma(anon_vma);
list_del(&avc->same_vma);
--
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