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Date: Wed, 31 Jan 2024 12:29:37 -0500
From: Gregory Price <gregory.price@memverge.com>
To: "Huang, Ying" <ying.huang@intel.com>
Cc: Gregory Price <gourry.memverge@gmail.com>, linux-mm@kvack.org,
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	Srinivasulu Thanneeru <sthanneeru.opensrc@micron.com>
Subject: Re: [PATCH v4 3/3] mm/mempolicy: introduce MPOL_WEIGHTED_INTERLEAVE
 for weighted interleaving
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On Wed, Jan 31, 2024 at 05:19:51PM +0800, Huang, Ying wrote:
> Gregory Price <gregory.price@memverge.com> 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;