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Message-ID: <871q9xeyo4.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Thu, 01 Feb 2024 09:55:07 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Gregory Price <gregory.price@...verge.com>
Cc: Gregory Price <gourry.memverge@...il.com>, <linux-mm@...ck.org>,
<linux-kernel@...r.kernel.org>, <linux-doc@...r.kernel.org>,
<linux-fsdevel@...r.kernel.org>, <linux-api@...r.kernel.org>,
<corbet@....net>, <akpm@...ux-foundation.org>, <honggyu.kim@...com>,
<rakie.kim@...com>, <hyeongtak.ji@...com>, <mhocko@...nel.org>,
<vtavarespetr@...ron.com>, <jgroves@...ron.com>,
<ravis.opensrc@...ron.com>, <sthanneeru@...ron.com>,
<emirakhur@...ron.com>, <Hasan.Maruf@....com>,
<seungjun.ha@...sung.com>, <hannes@...xchg.org>,
<dan.j.williams@...el.com>, Srinivasulu Thanneeru
<sthanneeru.opensrc@...ron.com>
Subject: Re: [PATCH v4 3/3] mm/mempolicy: introduce MPOL_WEIGHTED_INTERLEAVE
for weighted interleaving
Gregory Price <gregory.price@...verge.com> writes:
> On Wed, Jan 31, 2024 at 05:19:51PM +0800, Huang, Ying wrote:
>> Gregory Price <gregory.price@...verge.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;
The __rcu usage can be checked with `sparse` directly. For example,
make C=1 mm/mempolicy.o
More details can be found in
https://www.kernel.org/doc/html/latest/dev-tools/sparse.html
Per my understanding, we shouldn't use "__rcu" here. Please search
"__rcu" in the following document.
https://www.kernel.org/doc/html/latest/RCU/checklist.html
> 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 */
~~~~~~~
depleted?
> 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);
node will be changed in the loop. So we need to change the logic here.
> - /* 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;
>
> +
Change by accident?
> 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;
--
Best Regards,
Huang, Ying
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