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bh=wztWl+3aVrB9ZaSvZauY3Y92cOMYfXSxVG3uQ3w3yp8=; b=l55emC2wodh8Y5v0NiRnqfhHfZceI8HYXOZY/mhTbQKS9e8Ud9CnsI46RjzI2nfBmnAi6R 69HXePqmsAuaEPDQ== Received: from imap1.dmz-prg2.suse.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by imap1.dmz-prg2.suse.org (Postfix) with ESMTPS id 9B9F83EA63; 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Spamd-Result: default: False [-3.01 / 50.00]; BAYES_HAM(-3.00)[100.00%]; SUSPICIOUS_RECIPS(1.50)[]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; FREEMAIL_TO(0.00)[gmail.com]; ARC_NA(0.00)[]; FUZZY_RATELIMITED(0.00)[rspamd.com]; RCPT_COUNT_TWELVE(0.00)[12]; MIME_TRACE(0.00)[0:+]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; RCVD_TLS_ALL(0.00)[]; DKIM_TRACE(0.00)[suse.cz:+]; RCVD_COUNT_TWO(0.00)[2]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:helo,imap1.dmz-prg2.suse.org:rdns,suse.cz:email,suse.cz:dkim,suse.cz:mid]; DWL_DNSWL_BLOCKED(0.00)[suse.cz:dkim]; MID_RHS_MATCH_FROM(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; TAGGED_RCPT(0.00)[]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from] X-Rspamd-Action: no action X-Rspamd-Server: rspamd2.dmz-prg2.suse.org X-Spam-Level: X-Rspamd-Queue-Id: C57205BCF8 X-Spam-Flag: NO X-Spam-Score: -3.01 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