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Wed, 04 Nov 2026 03:11:26 -0800 (PST) Received: from milan ([2001:9b1:d5a0:a500::24b]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-c4d0e4a4bb5sm55813666b.80.2026.11.04.03.11.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 04 Nov 2026 03:11:26 -0800 (PST) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Wed, 4 Nov 2026 12:11:24 +0100 To: Herbert Xu Cc: Linus Torvalds , "Liam R. Howlett (Google)" , Davidlohr Bueso , Uladzislau Rezki , Michal Hocko , Baoquan He , Alexander Potapenko , Andrey Ryabinin , Marco Elver , Michal Hocko , Linux Crypto Mailing List , Andrew Morton , Thomas Graf , linux-mm@kvack.org, linux-kernel@vger.kernel.org, maple-tree@lists.infradead.org, Lorenzo Stoakes , Chris Mason , Alice Ryhl , Andrew Ballance , stable@vger.kernel.org, "David Hildenbrand (Arm)" Subject: Re: [PATCH 4/4] rhashtable: fix vmalloc() deadlock in atomic rehash Message-ID: References: X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Hello, Herbert! > > > > None of that says why. It only repeats what I already said: it seems > > to be due to kmsan doing bad things. > > Fair enough. > > When I was told that folks needed vmalloc under GFP_ATOMIC I didn't > question it any further: > > https://lore.kernel.org/all/aeiHkG7X_dieMdw5@milan/ > > Looking back, it does look like it was never explained who these > folks are. > > Ulazislau, so who needs vmalloc GFP_ATOMIC support in the kernel? > Or is it only used by out-of-kernel code? > There are users in kernel. For example: drivers/GPU /* Allocate the file in vmalloc memory, it's likely to be big */ iter.start = __vmalloc(file_size, GFP_NOWAIT); AMD-GPU: kvzalloc(size, GFP_NOWAIT); kvzalloc_objs(struct amdgpu_coredump_ring, GFP_NOWAIT); kvzalloc(ring->ring_size, GFP_NOWAIT); Virtio ring-testing: (alloc_ring(0)), gfpflags_allow_blocking(0) does not allow blocking. net/ipv4/udp.c: err = bpf_iter_udp_realloc_batch(iter, batch_sks, GFP_NOWAIT); etc... Apart of above, the obvious user of GFP_ATOMIC is the bridge forwarding database. The br_fdb_add_local() holds br->hash_lock with bottom halves disabled, on rehash event it invokes rhashtable_insert_rehash() to allocate a new bucket table with GFP_ATOMIC | __GFP_NOWARN. In that scenario kvmalloc() can fallback to vmalloc(). This path runs and exists because syzbot hit the old in_interrupt() check there, leading to 04aa71da5f3 ("mm/vmalloc: do not trigger BUG() on BH disabled context"). The vmalloc() fallback improves the chance of completing the insert on the spot to minimize -ENOMEM hit rate thus minimize queuing the irq_work_queue(&ht->run_irq_work) to retry allocation in process context. According to XFS code and comments it wants non-blocking support for kvmalloc(). But indeed the v4 cover letter didn't name a consumer. The bridge report came later. -- Uladzislau Rezki