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bh=GUvXlGSR5dfv4DsHE/Q9B87BUvBYICqk18OVwp4dZpo=; b=oFRwmpdDQn5NL2xshmLSPbX2C962zstdmxgel3QZFZ5YhJLiZ3HFcE1fxDHI7+KICYT2TN +AYEWJOLvuS7y1Dw== 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 8BAC11374A; Mon, 13 Oct 2025 12:58:54 +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 DuqtIY737GgQPwAAD6G6ig (envelope-from ); Mon, 13 Oct 2025 12:58:54 +0000 Message-ID: Date: Mon, 13 Oct 2025 14:58:54 +0200 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 v2 2/2] mm/page_alloc: Prevent reporting pcp->batch = 0 Content-Language: en-US To: Joshua Hahn , Andrew Morton Cc: Dave Hansen , Brendan Jackman , Johannes Weiner , Michal Hocko , Suren Baghdasaryan , Zi Yan , linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel-team@meta.com References: <20251009192933.3756712-1-joshua.hahnjy@gmail.com> <20251009192933.3756712-3-joshua.hahnjy@gmail.com> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Spam-Level: X-Spam-Flag: NO X-Rspamd-Queue-Id: A355D21A0C X-Rspamd-Action: no action X-Rspamd-Server: rspamd1.dmz-prg2.suse.org 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]; FUZZY_RATELIMITED(0.00)[rspamd.com]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; TO_DN_SOME(0.00)[]; FREEMAIL_TO(0.00)[gmail.com,linux-foundation.org]; MIME_TRACE(0.00)[0:+]; ARC_NA(0.00)[]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; RCVD_TLS_ALL(0.00)[]; DKIM_TRACE(0.00)[suse.cz:+]; RCVD_COUNT_TWO(0.00)[2]; DNSWL_BLOCKED(0.00)[2a07:de40:b281:104:10:150:64:97:from,2a07:de40:b281:106:10:150:64:167:received]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from]; MID_RHS_MATCH_FROM(0.00)[]; RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:106:10:150:64:167:received]; RCPT_COUNT_SEVEN(0.00)[11]; RCVD_VIA_SMTP_AUTH(0.00)[]; TAGGED_RCPT(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo,suse.cz:dkim,suse.cz:mid,suse.cz:email] X-Spam-Score: -3.01 On 10/9/25 21:29, Joshua Hahn wrote: > zone_batchsize returns the appropriate value that should be used for > pcp->batch. If it finds a zone with less than 4096 pages or PAGE_SIZE > > 1M, however, it leads to some incorrect math. > > In the above case, we will get an intermediary value of 1, which is then > rounded down to the nearest power of two, and 1 is subtracted from it. > Since 1 is already a power of two, we will get batch = 1-1 = 0: > > batch = rounddown_pow_of_two(batch + batch/2) - 1; > > A pcp->batch value of 0 is nonsensical. If this were actually set, then > functions like drain_zone_pages would become no-ops, since they could > only free 0 pages at a time. > > Of the two callers of zone_batchsize, the one that is actually used to > set pcp->batch works around this by setting pcp->batch to the maximum > of 1 and zone_batchsize. However, the other caller, zone_pcp_init, > incorrectly prints out the batch size of the zone to be 0. > > This is probably rare in a typical zone, but the DMA zone can often have > less than 4096 pages, which means it will print out "LIFO batch:0". > > Before: [ 0.001216] DMA zone: 3998 pages, LIFO batch:0 > After: [ 0.001210] DMA zone: 3998 pages, LIFO batch:1 > > Instead of dealing with the error handling and the mismatch between the > reported and actual zone batchsize, just return 1 if the zone_batchsize > is 1 page or less before the rounding. > > Signed-off-by: Joshua Hahn Acked-by: Vlastimil Babka