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t=1789574247; bh=XN+Zp9/GNf101whKzUoJhTi8UW65xULFMuNHBkfGcuA=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=CPmWOdUcWYOk0Bg/VDgSvDyRu5zASVMzHcID6o5azxbk2nZhuxHgyNYA+Ppz52ohF ZxqHXao6kQZKEGDPkED8gFUGKQwrYGtGsRMNmH6LhdXLgbWFu28O/mjS0Mp4z+7zqc Eh2Uh+8BKqAs0R55QoRsfjjqwZZtGh3yZQk/IyPnN3azypSr1NlDux0C+635plufhJ Wi5m8WQohlZVEavxVQ1Rq2xr4yJP8/rI+ANbT/P0pUQ+bm6J+klXikzHldtaY0md8m IIahutEc8ADm6yAURbBhRcBF6CHpDx4r+ygMjo6mlM6NJd1WDT8F77cHs09RHsULcb MW6wpuv6VUK3g== Message-ID: Date: Wed, 16 Sep 2026 17:57:22 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] mm/mempolicy: stop copying the nodemask in the interleave paths To: Gregory Price , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, akpm@linux-foundation.org, ziy@nvidia.com, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, ying.huang@linux.alibaba.com, apopple@nvidia.com, urezki@gmail.com, chenwandun@huawei.com References: <20260829015943.1258774-1-gourry@gourry.net> <20260829015943.1258774-3-gourry@gourry.net> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: A98AEC0007 X-Rspam-User: X-Stat-Signature: jykphgkwodd63xtprx7ty96o5gep98n1 X-HE-Tag: 1789574248-903401 X-HE-Meta: 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 T6Iy8eq4 InkjUrqmOUviGoIfGygFU/FCm0y7lrugEWKmOVmm5FXGdMgmyY75lJ0Urckss88GRQrxwSsupNQinLO615QkorwwV3hLZ2Y76dXCouCSOwBZRD0iGpWorVKDHegAUby6PKnCkWuS6oQtoe/K0tr95wdT/I5oUn+RovUaKt3/gz+TPSFja3m3PH8DgXljGWitBon3fuZBeE8ah+u4oWi7Y0uteV7fSyXBRN/kGXINHCAril5ZFvXpMRal+SkaNAnpGR0les1EQhanrBqnY7XkscBda6eqx8Z6ccNvtVjmk+atwQqLpEgYwrzMTtCKKYiv+SsDjY5EyN0qEtong9mwkqZLQTZWRhYwBj5yHwnT41eNDcAg= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/29/26 03:59, Gregory Price wrote: > The interleave node selectors copy pol->nodes onto the stack so the mask > cannot change while they walk it. nodemask_t is 128 bytes at > MAX_NUMNODES=1024, and two of the three run per folio fault. > > The copy only buys consistency between the node count and the walk. > Drop the consistency and just bounds check the walk instead. I think you should document here that accessing the node bitnmap is safe (is RCU responsible for that? I think yes), but it can get updated concurrently. > > If an empty nodelist or weight is perceived, fall back to numa_node_id(), > which is what the functions already did when the copy came back empty. Makes sense. > > weighted_interleave_nid() counts the nodes as we sum the weights. We use > that node count to limit the maximum skew a single node can host. > > interleave_nid() walks with next_node_in() rather than next_node(), so a > mask that shrank mid-walk wraps to a node still in the policy. > > alloc_pages_bulk_weighted_interleave() derives per-node counts from a > weight total summed over the mask, so a changing mask can make them exceed > the request. Clamp each chunk to the space left in page_array. > > A cpuset cookie will not work here: two of these take VMA policies, which > mpol_rebind_mm() rebinds under mmap_write_lock(), not mems_allowed_seq. > > Cost is distribution accuracy during a rebind - but the copy never > corrected this anyway, it was just a safety mechanism to prevent div/0 > and overrunning the alloc request buffer. > > Remove read_once_policy_nodemask(), now unused. > > -fstack-usage at MAX_NUMNODES=1024: > > weighted_interleave_nid 184 -> 56 > interleave_nid 168 -> 32 > alloc_pages_bulk_mempolicy_noprof 360 -> 136 Nice! > static unsigned int weighted_interleave_nid(struct mempolicy *pol, pgoff_t ilx) > { > struct weighted_interleave_state *state; > - nodemask_t nodemask; > - unsigned int target, nr_nodes; > + unsigned int target, nnodes = 0; > u8 *table = NULL; > unsigned int weight_total = 0; > u8 weight; > int nid = 0; > > - nr_nodes = read_once_policy_nodemask(pol, &nodemask); > - if (!nr_nodes) > - return numa_node_id(); > - > rcu_read_lock(); > > state = rcu_dereference(wi_state); > @@ -2232,22 +2213,40 @@ static unsigned int weighted_interleave_nid(struct mempolicy *pol, pgoff_t ilx) > if (state) > table = state->iw_table; > > - /* calculate the total weight */ > - for_each_node_mask(nid, nodemask) > + /* calculate the total weight and the node count */ > + for_each_node_mask(nid, pol->nodes) { > weight_total += table ? table[nid] : 1; > + nnodes++; > + } > + > + /* the mask is empty */ > + if (!weight_total) { > + rcu_read_unlock(); > + return numa_node_id(); > + } > > /* Calculate the node offset based on totals */ > target = ilx % weight_total; > - nid = first_node(nodemask); > - while (target) { > + nid = first_node(pol->nodes); > + > + /* > + * The target was calculated in a separate loop, and a concurrent > + * rebind can change the total number of nodes. Clamp this loop to > + * a single pass (nnodes) to keep the walk bounded by node count. Might want to explicitly comment here that we are looking at a moving target and might race with node bitmap modifications. > + */ > + while (target && nnodes-- && nid < MAX_NUMNODES) { > /* detect system default usage */ > weight = table ? table[nid] : 1; > if (target < weight) > break; > target -= weight; > - nid = next_node_in(nid, nodemask); > + nid = next_node_in(nid, pol->nodes); > } > rcu_read_unlock(); > + > + /* the mask emptied under the walk */ > + if (nid >= MAX_NUMNODES) > + return numa_node_id(); Makes sense. > return nid; > } > > @@ -2258,18 +2257,21 @@ static unsigned int weighted_interleave_nid(struct mempolicy *pol, pgoff_t ilx) > */ > static unsigned int interleave_nid(struct mempolicy *pol, pgoff_t ilx) > { > - nodemask_t nodemask; > unsigned int target, nnodes; > int i; > int nid; > > - nnodes = read_once_policy_nodemask(pol, &nodemask); > + nnodes = nodes_weight(pol->nodes); > if (!nnodes) > return numa_node_id(); > target = ilx % nnodes; > - nid = first_node(nodemask); > - for (i = 0; i < target; i++) > - nid = next_node(nid, nodemask); > + nid = first_node(pol->nodes); Similarly, I wonder whether we should spell out the raciness. [...] Nothing jumped at me Acked-by: David Hildenbrand (Arm) -- Cheers, David