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Received: from willy by casper.infradead.org with local (Exim 4.99.1 #2 (Red Hat Linux)) id 1wyL0e-000000059Vk-0XNn; Mon, 24 Aug 2026 03:06:04 +0000 Date: Mon, 24 Aug 2026 04:06:03 +0100 From: Matthew Wilcox To: Gregory Price Cc: linux-mm@kvack.org, akpm@linux-foundation.org, edumazet@google.com, linux-kernel@vger.kernel.org, kernel-team@meta.com, stable@vger.kernel.org, david@kernel.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, syzbot+0dbf6d295b3350944f0b@syzkaller.appspotmail.com Subject: Re: [PATCH] mm/mempolicy: refcount the weighted interleave state instead of copying it Message-ID: References: <20260821104043.f692421fec915c0c5bc1fbe6@linux-foundation.org> <20260824024117.1755899-1-gourry@gourry.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260824024117.1755899-1-gourry@gourry.net> X-Stat-Signature: bei3ahbb1xa6wbjpfpa1u3wk6g6i5zwr X-Rspamd-Queue-Id: E123A40008 X-Rspamd-Server: rspam03 X-Rspam-User: X-HE-Tag: 1787540770-159216 X-HE-Meta: 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 D4/zoPIe xMZFOsTec88FrkW4fnlbIKKOTELEZvdeUex/H9TH+0oX2p+1yM2j8E7bXTixvQe8CLgAKKETkSrb/5tF9Hyimfh5RUgwc+aqgzq7UhsRpP7bZOFxMJhFXkjojt09tEs+RQrIMc20kBM+Sas33c37CAmFK697dpzfOlWgj7xzluGlovthgSdTpII3ABHB7dYQGqTgPx1jMKZZ/fHwz5eGVWPvXw1lrPt3vMqh61w33jqOeORypULFajHDXw1C0vMrOjf0v/AK4VakWZjFRpqLAs7Ak5+XNQsVex+7OEYYwYpZGNP78B5nvbZbbNL3Ds9UATOGjVspYDjBTu3vd/UVHLUlA51NLf5LNTCwYTwxL5QTvBEVErB+zXCuMLAAXjquSVI+2Yw891vVzw+jwWlJHFbz58A== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sun, Aug 23, 2026 at 10:41:17PM -0400, Gregory Price wrote: > @@ -2632,7 +2654,7 @@ static unsigned long alloc_pages_bulk_weighted_interleave(gfp_t gfp, > unsigned long nr_allocated = 0; > unsigned long rounds; > unsigned long node_pages, delta; > - u8 *weights, weight; > + u8 *table, weight; > unsigned int weight_total = 0; > unsigned long rem_pages = nr_pages; > nodemask_t nodes; > @@ -2676,25 +2698,12 @@ static unsigned long alloc_pages_bulk_weighted_interleave(gfp_t gfp, > me->il_weight = 0; > prev_node = node; > > - /* create a local copy of node weights to operate on outside rcu */ > - weights = kzalloc(nr_node_ids, GFP_KERNEL); > - if (!weights) > - return total_allocated; > - > - rcu_read_lock(); > - state = rcu_dereference(wi_state); > - if (state) { > - memcpy(weights, state->iw_table, nr_node_ids * sizeof(u8)); > - rcu_read_unlock(); > - } else { > - rcu_read_unlock(); > - for (i = 0; i < nr_node_ids; i++) > - weights[i] = 1; > - } > + state = wi_state_get(); > + table = state ? state->iw_table : NULL; > > /* calculate total, detect system default usage */ > for_each_node_mask(node, nodes) > - weight_total += weights[node]; > + weight_total += table ? table[node] : 1; > > /* > * Calculate rounds/partial rounds to minimize __alloc_pages_bulk calls. > @@ -2706,10 +2715,10 @@ static unsigned long alloc_pages_bulk_weighted_interleave(gfp_t gfp, > rounds = rem_pages / weight_total; > delta = rem_pages % weight_total; > resume_node = next_node_in(prev_node, nodes); > - resume_weight = weights[resume_node]; > + resume_weight = table ? table[resume_node] : 1; > for (i = 0; i < nnodes; i++) { > node = next_node_in(prev_node, nodes); > - weight = weights[node]; > + weight = table ? table[node] : 1; > node_pages = weight * rounds; > /* If a delta exists, add this node's portion of the delta */ > if (delta > weight) { > @@ -2735,7 +2744,7 @@ static unsigned long alloc_pages_bulk_weighted_interleave(gfp_t gfp, > } > me->il_prev = resume_node; > me->il_weight = resume_weight; > - kfree(weights); > + wi_state_put(state); > return total_allocated; > } > Bit of a shame to take/put a refcount on every memory allocation. That seems like it might hurt (even being on a different cacheline). Would it make more sense to use SRCU for this?