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[129.59.122.134]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-4f4ac62fa56sm21392181cf.17.2025.12.19.07.51.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 19 Dec 2025 07:51:57 -0800 (PST) Date: Fri, 19 Dec 2025 09:51:55 -0600 Message-ID: From: Hajime Tazaki To: daniel@0x0f.com Cc: joshua.hahnjy@gmail.com, akpm@linux-foundation.org, linux@roeck-us.net, jackmanb@google.com, hannes@cmpxchg.org, mhocko@suse.com, surenb@google.com, vbabka@suse.cz, ziy@nvidia.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel-team@meta.com Subject: Re: [PATCH v2] mm/page_alloc: Report 1 as zone_batchsize for !CONFIG_MMU In-Reply-To: References: <20251218083200.2435789-1-joshua.hahnjy@gmail.com> User-Agent: Wanderlust/2.15.9 (Almost Unreal) Emacs/26.3 Mule/6.0 MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=US-ASCII X-Rspam-User: X-Rspamd-Queue-Id: 9FE71180019 X-Rspamd-Server: rspam04 X-Stat-Signature: hhfeqguupww3g4z31hty14m8refko6kt X-HE-Tag: 1766159519-286397 X-HE-Meta: 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 ZOuWbWEu 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hello Daniel, On Thu, 18 Dec 2025 06:30:42 -0600, Daniel Palmer wrote: > > Hi Joshua, > > On Thu, 18 Dec 2025 at 17:32, Joshua Hahn wrote: > > > > Commit 2783088ef24e ("mm/page_alloc: prevent reporting pcp->batch = 0") > > moved the error handling (0-handling) of zone_batchsize from its > > callers to inside the function. However, the commit left out the error > > handling for the NOMMU case, leading to deadlocks on NOMMU systems. > > > > For NOMMU systems, return 1 instead of 0 for zone_batchsize, which restores > > the previous deadlock-free behavior. > > Tested this on my 68000 setup, filled the memory to cause an OOM and I > got OOM instead of deadlock as expected. > > Tested-by: Daniel Palmer > > FWIW There was a BoF about NOMMU at LPC last week and I did mention to > the people presenting that seem to be using NOMMU in real world > applications that NOMMU was broken in mainline. I hoped they would > have chimed in on this.. I tested with UML with nommu extension (currently out of kernel *1) and reproduced the issue with a crafted program causing OOM. without patch it indeed hangs up with losing console access and this patch fixes with a proper failure message like below; oom: page allocation failure: order:12, mode:0xcc0(GFP_KERNEL), nodemask=(null) CPU: 0 UID: 0 PID: 32 Comm: oom Not tainted 6.18.0-12966-gc43a4f128407-dirty #223 NONE Stack: 60a8fb80 604a246e 603b9569 00000001 ffffff00 604a246e 6002440d 604a1479 60a8fbb0 6002bbb3 60556910 00000000 Call Trace: [<6002440d>] ? _printk+0x0/0x5b [<6002df89>] show_stack+0x11c/0x12b [<603b9569>] ? dump_stack_print_info+0x0/0x12f [<6002440d>] ? _printk+0x0/0x5b [<6002bbb3>] dump_stack_lvl+0x65/0x80 [<6002bbec>] dump_stack+0x1e/0x20 [<600e0c13>] warn_alloc+0x118/0x195 [<60083ae0>] ? __mutex_trylock+0x16/0x1e (snip) Tested-by: Hajime Tazaki *1 https://lore.kernel.org/all/cover.1762588860.git.thehajime@gmail.com/ -- Hajime