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-rw-r--r--Documentation/vm/numa_memory_policy.txt68
-rw-r--r--include/linux/mempolicy.h35
-rw-r--r--ipc/shm.c5
-rw-r--r--mm/hugetlb.c2
-rw-r--r--mm/mempolicy.c146
-rw-r--r--mm/shmem.c16
6 files changed, 195 insertions, 77 deletions
diff --git a/Documentation/vm/numa_memory_policy.txt b/Documentation/vm/numa_memory_policy.txt
index 27b9507..6719d64 100644
--- a/Documentation/vm/numa_memory_policy.txt
+++ b/Documentation/vm/numa_memory_policy.txt
@@ -311,6 +311,74 @@ Components of Memory Policies
MPOL_PREFERRED policies that were created with an empty nodemask
(local allocation).
+MEMORY POLICY REFERENCE COUNTING
+
+To resolve use/free races, struct mempolicy contains an atomic reference
+count field. Internal interfaces, mpol_get()/mpol_put() increment and
+decrement this reference count, respectively. mpol_put() will only free
+the structure back to the mempolicy kmem cache when the reference count
+goes to zero.
+
+When a new memory policy is allocated, it's reference count is initialized
+to '1', representing the reference held by the task that is installing the
+new policy. When a pointer to a memory policy structure is stored in another
+structure, another reference is added, as the task's reference will be dropped
+on completion of the policy installation.
+
+During run-time "usage" of the policy, we attempt to minimize atomic operations
+on the reference count, as this can lead to cache lines bouncing between cpus
+and NUMA nodes. "Usage" here means one of the following:
+
+1) querying of the policy, either by the task itself [using the get_mempolicy()
+ API discussed below] or by another task using the /proc/<pid>/numa_maps
+ interface.
+
+2) examination of the policy to determine the policy mode and associated node
+ or node lists, if any, for page allocation. This is considered a "hot
+ path". Note that for MPOL_BIND, the "usage" extends across the entire
+ allocation process, which may sleep during page reclaimation, because the
+ BIND policy nodemask is used, by reference, to filter ineligible nodes.
+
+We can avoid taking an extra reference during the usages listed above as
+follows:
+
+1) we never need to get/free the system default policy as this is never
+ changed nor freed, once the system is up and running.
+
+2) for querying the policy, we do not need to take an extra reference on the
+ target task's task policy nor vma policies because we always acquire the
+ task's mm's mmap_sem for read during the query. The set_mempolicy() and
+ mbind() APIs [see below] always acquire the mmap_sem for write when
+ installing or replacing task or vma policies. Thus, there is no possibility
+ of a task or thread freeing a policy while another task or thread is
+ querying it.
+
+3) Page allocation usage of task or vma policy occurs in the fault path where
+ we hold them mmap_sem for read. Again, because replacing the task or vma
+ policy requires that the mmap_sem be held for write, the policy can't be
+ freed out from under us while we're using it for page allocation.
+
+4) Shared policies require special consideration. One task can replace a
+ shared memory policy while another task, with a distinct mmap_sem, is
+ querying or allocating a page based on the policy. To resolve this
+ potential race, the shared policy infrastructure adds an extra reference
+ to the shared policy during lookup while holding a spin lock on the shared
+ policy management structure. This requires that we drop this extra
+ reference when we're finished "using" the policy. We must drop the
+ extra reference on shared policies in the same query/allocation paths
+ used for non-shared policies. For this reason, shared policies are marked
+ as such, and the extra reference is dropped "conditionally"--i.e., only
+ for shared policies.
+
+ Because of this extra reference counting, and because we must lookup
+ shared policies in a tree structure under spinlock, shared policies are
+ more expensive to use in the page allocation path. This is expecially
+ true for shared policies on shared memory regions shared by tasks running
+ on different NUMA nodes. This extra overhead can be avoided by always
+ falling back to task or system default policy for shared memory regions,
+ or by prefaulting the entire shared memory region into memory and locking
+ it down. However, this might not be appropriate for all applications.
+
MEMORY POLICY APIs
Linux supports 3 system calls for controlling memory policy. These APIS
diff --git a/include/linux/mempolicy.h b/include/linux/mempolicy.h
index 017def8..172b9c6 100644
--- a/include/linux/mempolicy.h
+++ b/include/linux/mempolicy.h
@@ -112,6 +112,31 @@ static inline void mpol_put(struct mempolicy *pol)
__mpol_put(pol);
}
+/*
+ * Does mempolicy pol need explicit unref after use?
+ * Currently only needed for shared policies.
+ */
+static inline int mpol_needs_cond_ref(struct mempolicy *pol)
+{
+ return (pol && (pol->flags & MPOL_F_SHARED));
+}
+
+static inline void mpol_cond_put(struct mempolicy *pol)
+{
+ if (mpol_needs_cond_ref(pol))
+ __mpol_put(pol);
+}
+
+extern struct mempolicy *__mpol_cond_copy(struct mempolicy *tompol,
+ struct mempolicy *frompol);
+static inline struct mempolicy *mpol_cond_copy(struct mempolicy *tompol,
+ struct mempolicy *frompol)
+{
+ if (!frompol)
+ return frompol;
+ return __mpol_cond_copy(tompol, frompol);
+}
+
extern struct mempolicy *__mpol_dup(struct mempolicy *pol);
static inline struct mempolicy *mpol_dup(struct mempolicy *pol)
{
@@ -201,6 +226,16 @@ static inline void mpol_put(struct mempolicy *p)
{
}
+static inline void mpol_cond_put(struct mempolicy *pol)
+{
+}
+
+static inline struct mempolicy *mpol_cond_copy(struct mempolicy *to,
+ struct mempolicy *from)
+{
+ return from;
+}
+
static inline void mpol_get(struct mempolicy *pol)
{
}
diff --git a/ipc/shm.c b/ipc/shm.c
index 8d1b2c4..e636910 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -271,10 +271,9 @@ static struct mempolicy *shm_get_policy(struct vm_area_struct *vma,
if (sfd->vm_ops->get_policy)
pol = sfd->vm_ops->get_policy(vma, addr);
- else if (vma->vm_policy) {
+ else if (vma->vm_policy)
pol = vma->vm_policy;
- mpol_get(pol); /* get_vma_policy() expects this */
- }
+
return pol;
}
#endif
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 53afa8c..d36e1f1 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -116,7 +116,7 @@ static struct page *dequeue_huge_page_vma(struct vm_area_struct *vma,
break;
}
}
- mpol_put(mpol); /* unref if mpol !NULL */
+ mpol_cond_put(mpol);
return page;
}
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 78b18a6..a237295 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -241,6 +241,15 @@ static struct mempolicy *mpol_new(unsigned short mode, unsigned short flags,
return policy;
}
+/* Slow path of a mpol destructor. */
+void __mpol_put(struct mempolicy *p)
+{
+ if (!atomic_dec_and_test(&p->refcnt))
+ return;
+ p->mode = MPOL_DEFAULT;
+ kmem_cache_free(policy_cache, p);
+}
+
static void mpol_rebind_default(struct mempolicy *pol, const nodemask_t *nodes)
{
}
@@ -719,6 +728,7 @@ static long do_get_mempolicy(int *policy, nodemask_t *nmask,
get_zonemask(pol, nmask);
out:
+ mpol_cond_put(pol);
if (vma)
up_read(&current->mm->mmap_sem);
return err;
@@ -1257,16 +1267,18 @@ asmlinkage long compat_sys_mbind(compat_ulong_t start, compat_ulong_t len,
*
* Returns effective policy for a VMA at specified address.
* Falls back to @task or system default policy, as necessary.
- * Returned policy has extra reference count if shared, vma,
- * or some other task's policy [show_numa_maps() can pass
- * @task != current]. It is the caller's responsibility to
- * free the reference in these cases.
+ * Current or other task's task mempolicy and non-shared vma policies
+ * are protected by the task's mmap_sem, which must be held for read by
+ * the caller.
+ * Shared policies [those marked as MPOL_F_SHARED] require an extra reference
+ * count--added by the get_policy() vm_op, as appropriate--to protect against
+ * freeing by another task. It is the caller's responsibility to free the
+ * extra reference for shared policies.
*/
static struct mempolicy *get_vma_policy(struct task_struct *task,
struct vm_area_struct *vma, unsigned long addr)
{
struct mempolicy *pol = task->mempolicy;
- int shared_pol = 0;
if (vma) {
if (vma->vm_ops && vma->vm_ops->get_policy) {
@@ -1274,20 +1286,20 @@ static struct mempolicy *get_vma_policy(struct task_struct *task,
addr);
if (vpol)
pol = vpol;
- shared_pol = 1; /* if pol non-NULL, add ref below */
} else if (vma->vm_policy &&
vma->vm_policy->mode != MPOL_DEFAULT)
pol = vma->vm_policy;
}
if (!pol)
pol = &default_policy;
- else if (!shared_pol && pol != current->mempolicy)
- mpol_get(pol); /* vma or other task's policy */
return pol;
}
-/* Return a nodemask representing a mempolicy */
-static nodemask_t *nodemask_policy(gfp_t gfp, struct mempolicy *policy)
+/*
+ * Return a nodemask representing a mempolicy for filtering nodes for
+ * page allocation
+ */
+static nodemask_t *policy_nodemask(gfp_t gfp, struct mempolicy *policy)
{
/* Lower zones don't get a nodemask applied for MPOL_BIND */
if (unlikely(policy->mode == MPOL_BIND) &&
@@ -1298,8 +1310,8 @@ static nodemask_t *nodemask_policy(gfp_t gfp, struct mempolicy *policy)
return NULL;
}
-/* Return a zonelist representing a mempolicy */
-static struct zonelist *zonelist_policy(gfp_t gfp, struct mempolicy *policy)
+/* Return a zonelist indicated by gfp for node representing a mempolicy */
+static struct zonelist *policy_zonelist(gfp_t gfp, struct mempolicy *policy)
{
int nd;
@@ -1311,10 +1323,10 @@ static struct zonelist *zonelist_policy(gfp_t gfp, struct mempolicy *policy)
break;
case MPOL_BIND:
/*
- * Normally, MPOL_BIND allocations node-local are node-local
- * within the allowed nodemask. However, if __GFP_THISNODE is
- * set and the current node is part of the mask, we use the
- * the zonelist for the first node in the mask instead.
+ * Normally, MPOL_BIND allocations are node-local within the
+ * allowed nodemask. However, if __GFP_THISNODE is set and the
+ * current node is part of the mask, we use the zonelist for
+ * the first node in the mask instead.
*/
nd = numa_node_id();
if (unlikely(gfp & __GFP_THISNODE) &&
@@ -1350,6 +1362,10 @@ static unsigned interleave_nodes(struct mempolicy *policy)
/*
* Depending on the memory policy provide a node from which to allocate the
* next slab entry.
+ * @policy must be protected by freeing by the caller. If @policy is
+ * the current task's mempolicy, this protection is implicit, as only the
+ * task can change it's policy. The system default policy requires no
+ * such protection.
*/
unsigned slab_node(struct mempolicy *policy)
{
@@ -1435,43 +1451,27 @@ static inline unsigned interleave_nid(struct mempolicy *pol,
* @mpol = pointer to mempolicy pointer for reference counted mempolicy
* @nodemask = pointer to nodemask pointer for MPOL_BIND nodemask
*
- * Returns a zonelist suitable for a huge page allocation.
- * If the effective policy is 'BIND, returns pointer to local node's zonelist,
- * and a pointer to the mempolicy's @nodemask for filtering the zonelist.
- * If it is also a policy for which get_vma_policy() returns an extra
- * reference, we must hold that reference until after the allocation.
- * In that case, return policy via @mpol so hugetlb allocation can drop
- * the reference. For non-'BIND referenced policies, we can/do drop the
- * reference here, so the caller doesn't need to know about the special case
- * for default and current task policy.
+ * Returns a zonelist suitable for a huge page allocation and a pointer
+ * to the struct mempolicy for conditional unref after allocation.
+ * If the effective policy is 'BIND, returns a pointer to the mempolicy's
+ * @nodemask for filtering the zonelist.
*/
struct zonelist *huge_zonelist(struct vm_area_struct *vma, unsigned long addr,
gfp_t gfp_flags, struct mempolicy **mpol,
nodemask_t **nodemask)
{
- struct mempolicy *pol = get_vma_policy(current, vma, addr);
struct zonelist *zl;
- *mpol = NULL; /* probably no unref needed */
+ *mpol = get_vma_policy(current, vma, addr);
*nodemask = NULL; /* assume !MPOL_BIND */
- if (pol->mode == MPOL_BIND) {
- *nodemask = &pol->v.nodes;
- } else if (pol->mode == MPOL_INTERLEAVE) {
- unsigned nid;
-
- nid = interleave_nid(pol, vma, addr, HPAGE_SHIFT);
- if (unlikely(pol != &default_policy &&
- pol != current->mempolicy))
- __mpol_put(pol); /* finished with pol */
- return node_zonelist(nid, gfp_flags);
- }
- zl = zonelist_policy(GFP_HIGHUSER, pol);
- if (unlikely(pol != &default_policy && pol != current->mempolicy)) {
- if (pol->mode != MPOL_BIND)
- __mpol_put(pol); /* finished with pol */
- else
- *mpol = pol; /* unref needed after allocation */
+ if (unlikely((*mpol)->mode == MPOL_INTERLEAVE)) {
+ zl = node_zonelist(interleave_nid(*mpol, vma, addr,
+ HPAGE_SHIFT), gfp_flags);
+ } else {
+ zl = policy_zonelist(gfp_flags, *mpol);
+ if ((*mpol)->mode == MPOL_BIND)
+ *nodemask = &(*mpol)->v.nodes;
}
return zl;
}
@@ -1526,25 +1526,23 @@ alloc_page_vma(gfp_t gfp, struct vm_area_struct *vma, unsigned long addr)
unsigned nid;
nid = interleave_nid(pol, vma, addr, PAGE_SHIFT);
- if (unlikely(pol != &default_policy &&
- pol != current->mempolicy))
- __mpol_put(pol); /* finished with pol */
+ mpol_cond_put(pol);
return alloc_page_interleave(gfp, 0, nid);
}
- zl = zonelist_policy(gfp, pol);
- if (pol != &default_policy && pol != current->mempolicy) {
+ zl = policy_zonelist(gfp, pol);
+ if (unlikely(mpol_needs_cond_ref(pol))) {
/*
- * slow path: ref counted policy -- shared or vma
+ * slow path: ref counted shared policy
*/
struct page *page = __alloc_pages_nodemask(gfp, 0,
- zl, nodemask_policy(gfp, pol));
+ zl, policy_nodemask(gfp, pol));
__mpol_put(pol);
return page;
}
/*
* fast path: default or task policy
*/
- return __alloc_pages_nodemask(gfp, 0, zl, nodemask_policy(gfp, pol));
+ return __alloc_pages_nodemask(gfp, 0, zl, policy_nodemask(gfp, pol));
}
/**
@@ -1574,10 +1572,15 @@ struct page *alloc_pages_current(gfp_t gfp, unsigned order)
cpuset_update_task_memory_state();
if (!pol || in_interrupt() || (gfp & __GFP_THISNODE))
pol = &default_policy;
+
+ /*
+ * No reference counting needed for current->mempolicy
+ * nor system default_policy
+ */
if (pol->mode == MPOL_INTERLEAVE)
return alloc_page_interleave(gfp, order, interleave_nodes(pol));
return __alloc_pages_nodemask(gfp, order,
- zonelist_policy(gfp, pol), nodemask_policy(gfp, pol));
+ policy_zonelist(gfp, pol), policy_nodemask(gfp, pol));
}
EXPORT_SYMBOL(alloc_pages_current);
@@ -1605,6 +1608,28 @@ struct mempolicy *__mpol_dup(struct mempolicy *old)
return new;
}
+/*
+ * If *frompol needs [has] an extra ref, copy *frompol to *tompol ,
+ * eliminate the * MPOL_F_* flags that require conditional ref and
+ * [NOTE!!!] drop the extra ref. Not safe to reference *frompol directly
+ * after return. Use the returned value.
+ *
+ * Allows use of a mempolicy for, e.g., multiple allocations with a single
+ * policy lookup, even if the policy needs/has extra ref on lookup.
+ * shmem_readahead needs this.
+ */
+struct mempolicy *__mpol_cond_copy(struct mempolicy *tompol,
+ struct mempolicy *frompol)
+{
+ if (!mpol_needs_cond_ref(frompol))
+ return frompol;
+
+ *tompol = *frompol;
+ tompol->flags &= ~MPOL_F_SHARED; /* copy doesn't need unref */
+ __mpol_put(frompol);
+ return tompol;
+}
+
static int mpol_match_intent(const struct mempolicy *a,
const struct mempolicy *b)
{
@@ -1639,15 +1664,6 @@ int __mpol_equal(struct mempolicy *a, struct mempolicy *b)
}
}
-/* Slow path of a mpol destructor. */
-void __mpol_put(struct mempolicy *p)
-{
- if (!atomic_dec_and_test(&p->refcnt))
- return;
- p->mode = MPOL_DEFAULT;
- kmem_cache_free(policy_cache, p);
-}
-
/*
* Shared memory backing store policy support.
*
@@ -2081,11 +2097,7 @@ int show_numa_map(struct seq_file *m, void *v)
pol = get_vma_policy(priv->task, vma, vma->vm_start);
mpol_to_str(buffer, sizeof(buffer), pol);
- /*
- * unref shared or other task's mempolicy
- */
- if (pol != &default_policy && pol != current->mempolicy)
- __mpol_put(pol);
+ mpol_cond_put(pol);
seq_printf(m, "%08lx %s", vma->vm_start, buffer);
diff --git a/mm/shmem.c b/mm/shmem.c
index 5326876..0b591c66 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1187,16 +1187,19 @@ static void shmem_show_mpol(struct seq_file *seq, unsigned short policy,
static struct page *shmem_swapin(swp_entry_t entry, gfp_t gfp,
struct shmem_inode_info *info, unsigned long idx)
{
+ struct mempolicy mpol, *spol;
struct vm_area_struct pvma;
struct page *page;
+ spol = mpol_cond_copy(&mpol,
+ mpol_shared_policy_lookup(&info->policy, idx));
+
/* Create a pseudo vma that just contains the policy */
pvma.vm_start = 0;
pvma.vm_pgoff = idx;
pvma.vm_ops = NULL;
- pvma.vm_policy = mpol_shared_policy_lookup(&info->policy, idx);
+ pvma.vm_policy = spol;
page = swapin_readahead(entry, gfp, &pvma, 0);
- mpol_put(pvma.vm_policy);
return page;
}
@@ -1204,16 +1207,17 @@ static struct page *shmem_alloc_page(gfp_t gfp,
struct shmem_inode_info *info, unsigned long idx)
{
struct vm_area_struct pvma;
- struct page *page;
/* Create a pseudo vma that just contains the policy */
pvma.vm_start = 0;
pvma.vm_pgoff = idx;
pvma.vm_ops = NULL;
pvma.vm_policy = mpol_shared_policy_lookup(&info->policy, idx);
- page = alloc_page_vma(gfp, &pvma, 0);
- mpol_put(pvma.vm_policy);
- return page;
+
+ /*
+ * alloc_page_vma() will drop the shared policy reference
+ */
+ return alloc_page_vma(gfp, &pvma, 0);
}
#else /* !CONFIG_NUMA */
#ifdef CONFIG_TMPFS
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