Re: [PATCH v4 3/3] mm/mempolicy: introduce MPOL_WEIGHTED_INTERLEAVE for weighted interleaving
From: Gregory Price
Date: Wed Jan 31 2024 - 12:30:08 EST
On Wed, Jan 31, 2024 at 05:19:51PM +0800, Huang, Ying wrote:
> Gregory Price <gregory.price@xxxxxxxxxxxx> writes:
>
> >
> > I think this is handled already? It is definitely an explicit race
> > condition that is documented elsewhere:
> >
> > /*
> > * mpol_rebind_policy - Migrate a policy to a different set of nodes
> > *
> > * Per-vma policies are protected by mmap_lock. Allocations using per-task
> > * policies are protected by task->mems_allowed_seq to prevent a premature
> > * OOM/allocation failure due to parallel nodemask modification.
> > */
>
> Thanks for pointing this out!
>
> If we use task->mems_allowed_seq reader side in
> weighted_interleave_nodes() we can guarantee the consistency of
> policy->nodes. That may be not deserved, because it's not a big deal to
> allocate 1 page in a wrong node.
>
> It makes more sense to do that in
> alloc_pages_bulk_array_weighted_interleave(), because a lot of pages may
> be allocated there.
>
To save the versioning if there are issues, here are the 3 diffs that
I have left. If you are good with these changes, I'll squash the first
2 into the third commit, keep the last one as a separate commit (it
changes the interleave_nodes() logic too), and submit v5 w/ your
reviewed tag on all of them.
Fix one (pedantic?) warning from syzbot:
----------------------------------------
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index b1437396c357..dfd097009606 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2391,7 +2391,7 @@ static unsigned long alloc_pages_bulk_array_weighted_interleave(gfp_t gfp,
unsigned long nr_allocated = 0;
unsigned long rounds;
unsigned long node_pages, delta;
- u8 __rcu *table, *weights, weight;
+ u8 __rcu *table, __rcu *weights, weight;
unsigned int weight_total = 0;
unsigned long rem_pages = nr_pages;
nodemask_t nodes;
Simplifying resume_node/weight logic:
-------------------------------------
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 2c1aef8eab70..b0ca9bcdd64c 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2405,15 +2405,9 @@ static unsigned long alloc_pages_bulk_array_weighted_interleave(gfp_t gfp,
page_array += nr_allocated;
total_allocated += nr_allocated;
/* if that's all the pages, no need to interleave */
- if (rem_pages < weight) {
- /* stay on current node, adjust il_weight */
+ if (rem_pages <= weight) {
me->il_weight -= rem_pages;
return total_allocated;
- } else if (rem_pages == weight) {
- /* move to next node / weight */
- me->il_prev = next_node_in(node, nodes);
- me->il_weight = get_il_weight(me->il_prev);
- return total_allocated;
}
/* Otherwise we adjust remaining pages, continue from there */
rem_pages -= weight;
@@ -2460,17 +2454,10 @@ static unsigned long alloc_pages_bulk_array_weighted_interleave(gfp_t gfp,
node_pages += weight;
delta -= weight;
} else if (delta) {
+ /* when delta is deleted, resume from that node */
node_pages += delta;
- /* delta may deplete on a boundary or w/ a remainder */
- if (delta == weight) {
- /* boundary: resume from next node/weight */
- resume_node = next_node_in(node, nodes);
- resume_weight = weights[resume_node];
- } else {
- /* remainder: resume this node w/ remainder */
- resume_node = node;
- resume_weight = weight - delta;
- }
+ resume_node = node;
+ resume_weight = weight - delta;
delta = 0;
}
/* node_pages can be 0 if an allocation fails and rounds == 0 */
task->mems_allowed_seq protection (added as 4th patch)
------------------------------------------------------
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index b0ca9bcdd64c..b1437396c357 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1879,10 +1879,15 @@ bool apply_policy_zone(struct mempolicy *policy, enum zone_type zone)
static unsigned int weighted_interleave_nodes(struct mempolicy *policy)
{
unsigned int node = current->il_prev;
+ unsigned int cpuset_mems_cookie;
+retry:
+ /* to prevent miscount use tsk->mems_allowed_seq to detect rebind */
+ cpuset_mems_cookie = read_mems_allowed_begin();
if (!current->il_weight || !node_isset(node, policy->nodes)) {
node = next_node_in(node, policy->nodes);
- /* can only happen if nodemask is being rebound */
+ if (read_mems_allowed_retry(cpuset_mems_cookie))
+ goto retry;
if (node == MAX_NUMNODES)
return node;
current->il_prev = node;
@@ -1896,10 +1901,17 @@ static unsigned int weighted_interleave_nodes(struct mempolicy *policy)
static unsigned int interleave_nodes(struct mempolicy *policy)
{
unsigned int nid;
+ unsigned int cpuset_mems_cookie;
+
+ /* to prevent miscount, use tsk->mems_allowed_seq to detect rebind */
+ do {
+ cpuset_mems_cookie = read_mems_allowed_begin();
+ nid = next_node_in(current->il_prev, policy->nodes);
+ } while (read_mems_allowed_retry(cpuset_mems_cookie));
- nid = next_node_in(current->il_prev, policy->nodes);
if (nid < MAX_NUMNODES)
current->il_prev = nid;
+
return nid;
}
@@ -2374,6 +2386,7 @@ static unsigned long alloc_pages_bulk_array_weighted_interleave(gfp_t gfp,
struct page **page_array)
{
struct task_struct *me = current;
+ unsigned int cpuset_mems_cookie;
unsigned long total_allocated = 0;
unsigned long nr_allocated = 0;
unsigned long rounds;
@@ -2388,10 +2401,17 @@ static unsigned long alloc_pages_bulk_array_weighted_interleave(gfp_t gfp,
int prev_node;
int i;
+
if (!nr_pages)
return 0;
- nnodes = read_once_policy_nodemask(pol, &nodes);
+ /* read the nodes onto the stack, retry if done during rebind */
+ do {
+ cpuset_mems_cookie = read_mems_allowed_begin();
+ nnodes = read_once_policy_nodemask(pol, &nodes);
+ } while (read_mems_allowed_retry(cpuset_mems_cookie));
+
+ /* if the nodemask has become invalid, we cannot do anything */
if (!nnodes)
return 0;