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X-CSE-ConnectionGUID: dk1B2pe+RJOmWaP/wutw3g== X-CSE-MsgGUID: DRDbDNAxQ7CCDDcCQ+mEFQ== X-IronPort-AV: E=McAfee;i="6800,10657,11710"; a="76722062" X-IronPort-AV: E=Sophos;i="6.21,306,1763452800"; d="scan'208";a="76722062" Received: from fmviesa005.fm.intel.com ([10.60.135.145]) by fmvoesa106.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 23 Feb 2026 07:31:07 -0800 X-CSE-ConnectionGUID: 3RdSeWRtQ1OGmwABwOqOQQ== X-CSE-MsgGUID: HpkJ5mxMT22hY3yqnfnyyg== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.21,306,1763452800"; d="scan'208";a="220144437" Received: from dnelso2-mobl.amr.corp.intel.com (HELO [10.125.110.205]) ([10.125.110.205]) by fmviesa005-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 23 Feb 2026 07:31:07 -0800 Message-ID: Date: Mon, 23 Feb 2026 07:31:08 -0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/1] mm/mmu_gather: replace IPI with synchronize_rcu() when batch allocation fails To: Lance Yang , david@kernel.org Cc: akpm@linux-foundation.org, aneesh.kumar@kernel.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, npiggin@gmail.com, peterz@infradead.org, will@kernel.org References: <2a6c4e62-1663-4a98-9adc-406a6a1ebfd3@kernel.org> <20260223125826.28207-1-lance.yang@linux.dev> Content-Language: en-US From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 2/23/26 04:58, Lance Yang wrote: ... > +/** > + * tlb_remove_table_sync_rcu() - synchronize with software page-table walkers > + * > + * Like tlb_remove_table_sync_one() but uses RCU grace period instead of IPI > + * broadcast. Should be used in slow paths where sleeping is acceptable. Just a nit on comments: Use imperative voice: ... Use in slow paths where sleeping is acceptable. > + * Software/Lockless page-table walkers use local_irq_disable(), which is also > + * an RCU read-side critical section. synchronize_rcu() waits for all such > + * sections, providing the same guarantee as tlb_remove_table_sync_one() but > + * without disrupting all CPUs with IPIs. Yep, synchronize_rcu() is likely slower (longer wall clock time) but less disruptive to other CPUs. Is it worth explaining here that this should be used when code really needs to _wait_ and *not* for freeing memory? Freeing memory should use RCU callbacks that don't cause latency spikes in this thread, not this. > + * Context: Can sleep/block. Cannot be called from any atomic context. As a general rule, expressing constraints like this is best done in code, not comments, so: might_sleep(); or WARN_ON_ONCE(!in_atomic()); seem appropriate. I didn't see any obvious warning like that in the top levels of synchronize_rcu(). > +static void tlb_remove_table_sync_rcu(void) > +{ > + synchronize_rcu(); > +} > + > #else /* !CONFIG_MMU_GATHER_RCU_TABLE_FREE */ > > static void tlb_remove_table_free(struct mmu_table_batch *batch) > @@ -303,6 +321,10 @@ static void tlb_remove_table_free(struct mmu_table_batch *batch) > __tlb_remove_table_free(batch); > } > > +static void tlb_remove_table_sync_rcu(void) > +{ > +} > + > #endif /* CONFIG_MMU_GATHER_RCU_TABLE_FREE */ This seems a _little_ dangerous to even define. We don't want this sneaking into use when it doesn't do anything.