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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=KUqg9k85; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2F47F606A31A; Tue, 27 Oct 2026 19:41:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=KUqg9k85 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4BE191F01825; Tue, 27 Oct 2026 19:41:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793130081; bh=0sFGvmgrvvdqPTtpY3ePN/YlR7rZ5HGLPPKOD8BQTbo=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=KUqg9k85dXedpUxZlyCn3ouG4WACyQDbub3inF4OVMYoH5feG4/NNiPbptLeRhegn SKw5wGbktOA7voLurZSAAgcuGFDFKzI0FM86U2UBPnHevtJL/EuTxW2KUq2JxeRzJN 5nmLyyESp6PMWhbXHarGTEtcNTKzLJNtMpjSX/IzCHpl7qWjhYf9YDX4fIHuu9q7XY mXGKpCuLXFlzWTVByl9zw3cGL7GPSTFExRV9Q1oPtXh+YFoKSgzPvy/Y9up7fXsUPY 3xntQC4/GuQ64zNpykYrG0yZFoewu/pYCR6VED14ZnMVxIwO2hUi86jK1fgzMFZNXa DjwyYVSOD4Jhw== Message-ID: <87e3d64e-8b25-4d51-a984-7ab8a58f785d@kernel.org> Date: Tue, 27 Oct 2026 20:41:02 +0100 X-Mailing-List: linux-cxl@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 32/46] mm/memory_hotplug: avoid races in pfn_to_online_page() with dying pgmap To: Lance Yang Cc: akpm@linux-foundation.org, hannes@cmpxchg.org, mhocko@kernel.org, qi.zheng@linux.dev, shakeel.butt@linux.dev, ljs@kernel.org, kasong@tencent.com, baohua@kernel.org, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, baoquan.he@linux.dev, baolin.wang@linux.alibaba.com, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, kees@kernel.org, peterz@infradead.org, jannh@google.com, pfalcato@suse.de, hughd@google.com, pasha.tatashin@soleen.com, pratyush@kernel.org, willy@infradead.org, jack@suse.cz, ziy@nvidia.com, riel@surriel.com, harry@kernel.org, apopple@nvidia.com, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, usama.arif@linux.dev, kas@kernel.org, matthew.brost@intel.com, jhubbard@nvidia.com, peterx@redhat.com, will@kernel.org, aneesh.kumar@kernel.org, npiggin@gmail.com, osalvador@suse.de, minchan@kernel.org, muchun.song@linux.dev, xu.xin@linux.dev, chengming.zhou@linux.dev, chrisl@kernel.org, shikemeng@huaweicloud.com, nphamcs@gmail.com, youngjun.park@lge.com, gourry@gourry.net, joshua.hahnjy@gmail.com, rakie.kim@sk.com, ying.huang@linux.alibaba.com, jgg@ziepe.ca, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-cxl@vger.kernel.org, josef@toxicpanda.com References: <20261027-fixes_core-v1-32-6f74bec9a5bb@kernel.org> <20261027121307.40431-1-lance.yang@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit > LLM stumbled upon a possible race in kpage_read() ... > > Say the reader passes pfn_section_valid() for a device PFN in a section > shared with online RAM, gets preempted, and resumes after > pgmap_array_delete(). We'd then find no pgmap and return pfn_to_page(pfn), > even though the subsection is no longer valid ... I think that's similar to our pfn_to_online_page() vs. memory offlining races, just slightly different. Getting these races down to 0 is pretty hard. This patch here tries to do better and arguably almost looks like a cleanup. I wouldn't worry about that one for now; it's extremely unlikely to hit. And it' > > Could we take the memory hotplug read lock in kpage_read(), covering both > the lookup and the metadata reads? We can drop it before put_user(), once > we're done with the page. > > Maybe along these lines? > > ---8<--- > diff --git a/fs/proc/page.c b/fs/proc/page.c > index 6aa99e42ec22..12bed36d76dc 100644 > --- a/fs/proc/page.c > +++ b/fs/proc/page.c > @@ -12,6 +12,7 @@ > #include > #include > #include > +#include > #include > #include > #include > @@ -77,6 +78,8 @@ static ssize_t kpage_read(struct file *file, char __user *buf, > count = min_t(unsigned long, count, (max_dump_pfn * KPMSIZE) - src); > > while (count > 0) { > + /* Prevent memory removal while inspecting the memmap. */ > + get_online_mems(); get_online_mems() is the big hammer we try to avoid (or ideally remove at some point). And it's most pfn_to_online_page() users that are affected. A seqcount in pfn_to_online_page() could detect concurrent pgmap removal and retry from the start. But I'd rather delay something like that. Thanks for taking a look! (Can I convince you to review some of the THP fixes? pleeeeaaasseee :) ) -- Cheers, David