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Fri, 17 Jul 2026 16:16:29 -0700 (PDT) Received: from localhost.localdomain ([107.198.84.185]) by smtp.gmail.com with ESMTPSA id 956f58d0204a3-6683b5645a3sm2597231d50.6.2026.07.17.16.16.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 17 Jul 2026 16:16:28 -0700 (PDT) From: Paul Sherman To: linux-mm@kvack.org Cc: tj@kernel.org, vbabka@suse.cz, dennis@kernel.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, Paul Sherman Subject: [PATCH] mm/percpu: allow embed allocator when total size fits in vmalloc Date: Fri, 17 Jul 2026 16:16:16 -0700 Message-ID: <20260717231616.9126-1-shermanpauldylan@gmail.com> X-Mailer: git-send-email 2.53.0 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 4928940006 X-Stat-Signature: d89kwsb7qu3dokb1ahtompmmdrsknqeq X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1784330190-121696 X-HE-Meta: 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 KrQgPdLf WDVJ+WFPKvCOgGpCfC/Kal3ptKDr3Pw7wq+jGirTEXDSz8AGyJmFtahqHCP897vUjWVogNwFHvNjQVYiJZfEoerMHQtHllipniKsP02P/vP8t0kv26Cc+BpvhwcTxoG6kM37Nt5A8Mbw8CRuiiDz1g4aT5n/eZYiezec3aZLR7SsC2UesSE3y/VKRf4PXb55R6USntlpp4kygaW9ed3NlZrWAxBasj4rTsSbq0HMj0/HyGJ4eaGP3rI20IRkZAvvFjbb9gC+FLD2nurTXAp38MOg+SnExtZ5IIPlxZCx88n5VI4rkGaCUF2ujBELYjdYYZ5FkxeIfQQ36S+OavoK5nx84T1M4EtZ5JU0SkM3YHbmZveAF6EsQxVsqtyWfnLtyC3xxKINLwjK47vi/bo9w6MEkwXrJRLJpQAxa4f7zdW2HzujknsFZcEBVoTZakzw9hU0dMeNSk7S7x1KodnmwJSSTGoBmL1ljBSGI0Supk/Cfs3w= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On large NUMA systems (e.g. 4-node 128GB), the physical span between NUMA nodes may exceed 75% of vmalloc space even though the total percpu allocation is tiny (e.g. ~6MB for 64 CPUs). The 75% check was designed for pcpu_get_vm_areas() used by dynamic percpu chunks, where vmalloc congruency is required. The embed allocator uses the linear map directly for the first chunk -- physical addresses are accessible as virtual addresses without vmalloc mapping. The relevant constraint is total percpu size, not physical span. Replace the unconditional fallback with a total-size check: if the aggregate percpu allocation fits within 75% of vmalloc space, proceed with embed regardless of physical NUMA span. Systems where total percpu size genuinely exceeds the vmalloc bound retain the page allocator fallback. Tested on Sophgo SG2042 (64-hart, 4-NUMA, 128GB DDR4, RISC-V Sv39): percpu: embed: span 0x17def7a000 > vmalloc 75% but total 0x5c0000 fits -- linear map used percpu: Embedded 23 pages/cpu s54168 r8192 d31848 u94208 Link: https://lkml.iu.edu/hypermail/linux/kernel/1707.3/00337.html Cc: Tejun Heo Cc: Vlastimil Babka Cc: Dennis Zhou Signed-off-by: Paul Sherman --- This fix is inert on systems where the original check never fired; it only activates when physical span exceeds 75% of vmalloc but total percpu size does not. Architecture analysis: Architecture | vmalloc | phys span | total percpu | result ---------------|--------------|--------------|--------------|-------- RISC-V Sv39 | ~88 GB | ~102 GB | ~6 MB | fixed RISC-V Sv48 | ~88 TB | ~102 GB | ~6 MB | unaffected RISC-V Sv57 | ~44 PB | ~102 GB | ~6 MB | unaffected ARM64 large | ~248 TB | varies | tiny | unaffected 32-bit NUMA | ~128 MB | varies | may exceed | correct fallback Tejun Heo noted in 2017 [Link] that the only constraint is vmalloc size relative to NUMA node distances, and that making vmalloc bigger would be the best fix. On RISC-V Sv39 with 88GB vmalloc and 102GB physical NUMA span that is not an option -- but the embed allocator does not need congruent vmalloc mapping for the first chunk anyway. mm/percpu.c | 29 ++++++++++++++++++++++++----- 1 file changed, 24 insertions(+), 5 deletions(-) diff --git a/mm/percpu.c b/mm/percpu.c index b0676b8054ed..72695778872c 100644 --- a/mm/percpu.c +++ b/mm/percpu.c @@ -3069,13 +3069,32 @@ int __init pcpu_embed_first_chunk(size_t reserved_size, size_t dyn_size, /* warn if maximum distance is further than 75% of vmalloc space */ if (max_distance > VMALLOC_TOTAL * 3 / 4) { - pr_warn("max_distance=0x%lx too large for vmalloc space 0x%lx\n", + pr_warn("percpu: embed: max_distance=0x%lx too large for vmalloc space 0x%lx\n", max_distance, VMALLOC_TOTAL); #ifdef CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK - /* and fail if we have fallback */ - rc = -EINVAL; - goto out_free_areas; -#endif + /* + * The embed allocator uses the linear map directly for the + * first chunk -- physical addresses are accessible as virtual + * addresses without vmalloc mapping. The 75% check was designed + * for pcpu_get_vm_areas() (dynamic chunks) where vmalloc + * congruency is required. On large NUMA systems, physical span + * between nodes may exceed vmalloc bounds even though total + * percpu size is tiny. Check total size, not physical span. + */ + { + unsigned long total_size = 0; + for (group = 0; group < ai->nr_groups; group++) + total_size += (unsigned long)ai->unit_size * + ai->groups[group].nr_units; + if (total_size > VMALLOC_TOTAL * 3 / 4) { + rc = -EINVAL; + goto out_free_areas; + } + pr_info("percpu: embed: span 0x%lx > vmalloc 75%%" + " but total 0x%lx fits -- linear map used\n", + max_distance, total_size); + } +#endif /* CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK */ } /* -- 2.53.0