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DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1784545971; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=MDxvOrx4DVj0VV+Y/wV9iOjtsl71lHDlM1UPvEJ30HQ=; b=ZpAtUeRq8aKa+BH6qVMMoCl8ndmZrk4t43VZkpCSSsf4Gkh9xQkZVmZdMY3bOD4eVC3BEC 2VWFSY6QKabWvc8Ax3La9JxEM63qz68wXHlsBAfIP4EXKGoSNo6c1LKcUOCZgebbcXmzzC 8pyU2R9O7vhHAM+z6TIsWuNURFOf/M8= From: Usama Arif To: Rik van Riel Cc: Usama Arif , linux-kernel@vger.kernel.org, Andrew Morton , kernel-team@meta.com, David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Usama Arif Subject: Re: [PATCH RFC v3 1/6] x86/mm: add untagged_addr_remote_unlocked() Date: Mon, 20 Jul 2026 04:12:37 -0700 Message-ID: <20260720111238.3313021-1-usama.arif@linux.dev> In-Reply-To: <20260717170036.743149-2-riel@surriel.com> References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: E07F64000B X-Stat-Signature: 9mdj3uxttyr96zm5sw9heg1t5fn86um1 X-HE-Tag: 1784545973-342564 X-HE-Meta: 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 C+K65uwW LbSR/1ZWALmXgpXsSBy9vZn+qxrmdxVatfTDEEdQ6giKAn98bX7rkCwcnf5kHSwqf6deDSKYFWGA0vPre9yoFXw5xR7DAUCglWEwepoq5bu/+OiCsvHMEmquVmOO+Q6p9uHWMJkwLCCrS1tyGxCfU+Zj3GFZiuCup1qgszUwxTuaAgX+59KBZS2rq6w4W18mGoEUmWXuNBU4r6TEsCITPCUt88gU6JqwBOab++w8L5Ki+GnDAiXoIemzTA4Rq9168arUsrbNN/kASHb5AWB5BRvK1itZ02H3K0mqoBOg+t7BTGEI4TDr6AdbKvGvqGxylMHxpvKUttRMKg4s= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, 17 Jul 2026 13:00:31 -0400 Rik van Riel wrote: > __access_remote_vm() reads another process's memory under the mmap lock. On > large machines that lock is contended by tasks polling /proc/PID/cmdline, > /proc/PID/environ, or calling process_vm_readv(), even though the memory > they read is almost always resident and could be reached under the per-VMA > lock instead. > > Looking up the VMA first requires untagging the address. > untagged_addr_remote() asserts the mmap lock only because it reads > mm->context.untag_mask, which can race with the write in mm_enable_lam(). > > That mask is set once, when LAM is enabled, and never changes afterwards, > so the read itself does not need the lock. It is already read without it, > from the context switch path and /proc/PID/status. > > Add untagged_addr_remote_unlocked() for callers that have not taken the > mmap lock, and annotate access to mm->context.untag_mask with READ_ONCE() > and WRITE_ONCE() so the existing lockless reads are explicit and > KCSAN-clean. untagged_addr_remote() keeps its assertion and shares the same > code. > > Assisted-by: Claude:claude-opus-4.8 > Acked-by: Usama Arif Hi Rik, Would you mind changing this to Acked-by: Usama Arif in the next revision Thanks!