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Wed, 17 Dec 2025 13:19:47 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id LO4MJPOtQmmLSgAAD6G6ig (envelope-from ); Wed, 17 Dec 2025 13:19:47 +0000 Message-ID: <17c45b15-326d-4b4a-b3f3-745002b81d5f@suse.cz> Date: Wed, 17 Dec 2025 14:19:47 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/page_alloc: restore 0-handling to zone_set_pageset_high_and_batch Content-Language: en-US To: Joshua Hahn Cc: Andrew Morton , Daniel Palmer , Guenter Roeck , Brendan Jackman , Johannes Weiner , Michal Hocko , Suren Baghdasaryan , Zi Yan , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Matthew Wilcox References: <20251217130322.3350377-1-joshua.hahnjy@gmail.com> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSBWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBzdXNlLmN6PsLBlAQTAQoAPgIbAwULCQgHAwUVCgkICwUWAgMBAAIe AQIXgBYhBKlA1DSZLC6OmRA9UCJPp+fMgqZkBQJnyBr8BQka0IFQAAoJECJPp+fMgqZkqmMQ AIbGN95ptUMUvo6aAdhxaOCHXp1DfIBuIOK/zpx8ylY4pOwu3GRe4dQ8u4XS9gaZ96Gj4bC+ jwWcSmn+TjtKW3rH1dRKopvC07tSJIGGVyw7ieV/5cbFffA8NL0ILowzVg8w1ipnz1VTkWDr 2zcfslxJsJ6vhXw5/npcY0ldeC1E8f6UUoa4eyoskd70vO0wOAoGd02ZkJoox3F5ODM0kjHu Y97VLOa3GG66lh+ZEelVZEujHfKceCw9G3PMvEzyLFbXvSOigZQMdKzQ8D/OChwqig8wFBmV QCPS4yDdmZP3oeDHRjJ9jvMUKoYODiNKsl2F+xXwyRM2qoKRqFlhCn4usVd1+wmv9iLV8nPs 2Db1ZIa49fJet3Sk3PN4bV1rAPuWvtbuTBN39Q/6MgkLTYHb84HyFKw14Rqe5YorrBLbF3rl M51Dpf6Egu1yTJDHCTEwePWug4XI11FT8lK0LNnHNpbhTCYRjX73iWOnFraJNcURld1jL1nV r/LRD+/e2gNtSTPK0Qkon6HcOBZnxRoqtazTU6YQRmGlT0v+rukj/cn5sToYibWLn+RoV1CE Qj6tApOiHBkpEsCzHGu+iDQ1WT0Idtdynst738f/uCeCMkdRu4WMZjteQaqvARFwCy3P/jpK uvzMtves5HvZw33ZwOtMCgbpce00DaET4y/UzsBNBFsZNTUBCACfQfpSsWJZyi+SHoRdVyX5 J6rI7okc4+b571a7RXD5UhS9dlVRVVAtrU9ANSLqPTQKGVxHrqD39XSw8hxK61pw8p90pg4G /N3iuWEvyt+t0SxDDkClnGsDyRhlUyEWYFEoBrrCizbmahOUwqkJbNMfzj5Y7n7OIJOxNRkB IBOjPdF26dMP69BwePQao1M8Acrrex9sAHYjQGyVmReRjVEtv9iG4DoTsnIR3amKVk6si4Ea X/mrapJqSCcBUVYUFH8M7bsm4CSxier5ofy8jTEa/CfvkqpKThTMCQPNZKY7hke5qEq1CBk2 wxhX48ZrJEFf1v3NuV3OimgsF2odzieNABEBAAHCwXwEGAEKACYCGwwWIQSpQNQ0mSwujpkQ PVAiT6fnzIKmZAUCZ8gcVAUJFhTonwAKCRAiT6fnzIKmZLY8D/9uo3Ut9yi2YCuASWxr7QQZ lJCViArjymbxYB5NdOeC50/0gnhK4pgdHlE2MdwF6o34x7TPFGpjNFvycZqccSQPJ/gibwNA zx3q9vJT4Vw+YbiyS53iSBLXMweeVV1Jd9IjAoL+EqB0cbxoFXvnjkvP1foiiF5r73jCd4PR rD+GoX5BZ7AZmFYmuJYBm28STM2NA6LhT0X+2su16f/HtummENKcMwom0hNu3MBNPUOrujtW khQrWcJNAAsy4yMoJ2Lw51T/5X5Hc7jQ9da9fyqu+phqlVtn70qpPvgWy4HRhr25fCAEXZDp xG4RNmTm+pqorHOqhBkI7wA7P/nyPo7ZEc3L+ZkQ37u0nlOyrjbNUniPGxPxv1imVq8IyycG AN5FaFxtiELK22gvudghLJaDiRBhn8/AhXc642/Z/yIpizE2xG4KU4AXzb6C+o7LX/WmmsWP Ly6jamSg6tvrdo4/e87lUedEqCtrp2o1xpn5zongf6cQkaLZKQcBQnPmgHO5OG8+50u88D9I rywqgzTUhHFKKF6/9L/lYtrNcHU8Z6Y4Ju/MLUiNYkmtrGIMnkjKCiRqlRrZE/v5YFHbayRD dJKXobXTtCBYpLJM4ZYRpGZXne/FAtWNe4KbNJJqxMvrTOrnIatPj8NhBVI0RSJRsbilh6TE m6M14QORSWTLRg== In-Reply-To: <20251217130322.3350377-1-joshua.hahnjy@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Action: no action X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 8AD208000A X-Stat-Signature: 1wg6hy3j68w7qng63mk4fq3hiyb64j4b X-HE-Tag: 1765977590-841985 X-HE-Meta: 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 HBqDkQT8 hhDKTmagcbuu3H30Fz0bhjJAq0zhtls2Gzmx0/mP4g0h1K/aG+mk/cLrtMEk4v8bGV+eZAReEK/vU231wHTIFKe2/nInn6nFMT8pvvJDEH0vi0WkJFt396qzMU1qS8fOmlZ/rwJalZnIZi8cFyqt20tz3+6Po9M6OF2rYh1NtpAHeL4BfK3kaZW9C8XgKL7K2tabH+ddf2Y2AKkJ6wmNa0jY9Gjdj1iSmdI/8pRsJA+OlB2ZB90HOuU6uV87srR8MuUWibqTLrHTe5jgQyiLc4Zk9dsCpPPkhNNRJ1DoAY8IZ0+rvpkrWv9T0sli4Shgm4i9OMau/WJPZz/QUvLumv72Ye4ViUcSHDKnl1V0ARYLXWL5ZSyQW0Int/ULkg88vBkqS6l1thdCsy2W669hNfSN61CXFqN3IZy4bEUFSbbeXNfIFpvn3V6T4WcAWY0+coshBMwp5kyk1yxM= 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: On 12/17/25 14:02, Joshua Hahn wrote: > On Wed, 17 Dec 2025 12:23:58 +0100 Vlastimil Babka wrote: > >> On 12/17/25 12:12, Vlastimil Babka wrote: >> > On 12/17/25 07:05, 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. >> >> >> >> Since in the NOMMU case the reported-to-user batchsize should still be 0, >> > >> > Should it? The value is effectively set to 1 despite what zone_batchsize() >> > returns, because of that adjustment this patch reinstates. Also does anyone >> > care, really? >> > >> >> we would only like the error handling to exist in the callsites that >> >> set the internal value for the zone (i.e. zone_set_pageset_high_and_batch). >> >> >> >> Restore max(1, zone_batchsize(zone)) to the callsite to prevent errors >> >> on NOMMU systems. >> > >> > I would rather make zone_batchsize() for !CONFIG_MMU return 1 instead of 0. >> >> Ah looks like you considered it too, initially: >> https://lore.kernel.org/all/20251211225947.822866-1-joshua.hahnjy@gmail.com/ >> >> It makes more sense to me than doing effectively two fixups in the MMU case. > > Hi Vlastimil, > > Thank you for your review as always. > > Yes, I had also considered returning 1 for the !MMU case, since I think it > would make it a lot simpler as well (It would also make my original patch > function as intended). > > However, I was unsure if changing this user-facing behavior for one line of > simplification would be worth it. I am not a NOMMU user, so I have very > little experience here, but I imagine that there is someone out there who > looks at zone_batchsize() returning 0 for NOMMU and interpreting it as > "there is no batching" as opposed to "there is batching, and it processes > 1 page at a time" (which, actually isn't even true anyways because of the > bitshift). Maybe an option is to just make batchsize not visible > in the NOMMU case in addition to always returning 1 to avoid confusion. > > Anyways, back to your original question of "does anyone care". . . > > I am not sure : -) It's a pr_debug(), it's not even being printed by default, nothing can possibly break by changing 0 to 1 there. So I really wouldn't overthink this... > For me, both solutions work, and in fact I prefer the original solution of > always reurning 1 for !NOMMU. Maybe some NOMMU users like Daniel and Guenter > can comment on whether this change really matters? I would be surprised if they were aware of that pr_debug() in the first place :) > Thank you again for your review and follow-up. I hope you have a great day! > Joshua