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b=lCRPp85gkA1330RWMyZ5bKiPPbXB4EYR3YGrg15qB9Mcsofye16ZLE8LLiHti+J2b Gk6uOrQuPqqE0Hi4sVFa72ObcJj97rZGClMsR3H5ZY0HtAc5alLIKDH+EmDeDd6Lw1 u+tljg+1TG6lDCMUQpxFY9RCsIcYJzgoCqzT+7rLGZCeKF7GMNXfhAt/1DttrMxapN mPBQrxOfz//7uxZsGlBTy2X2ROhTyHFMgIScEfxqEmC/nGcqCXp0kaXveTzkU0fFeV YnCKW6TKLJQnVOuPImnfnSSBtG1qow5vv/LxjvM8g8FPWzqFqN2mvRrjqCr3QVaUTL qrNdjxKDECKFg== Message-ID: <0ec17401-556d-4a85-9657-23a21aed09a2@kernel.org> Date: Tue, 20 Oct 2026 09:07:09 +0200 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/page_alloc: count guard pages as free again when their buddy merges To: Rik van Riel , linux-kernel@vger.kernel.org Cc: Andrew Morton , Suren Baghdasaryan , fvdl@google.com, Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , Kairui Song , Qi Zheng , Shakeel Butt , linux-mm@kvack.org, Baoquan He , David Hildenbrand , Lorenzo Stoakes , Dev Jain , stable@vger.kernel.org References: <20261016122304.5b1eba8e@fangorn> From: "Vlastimil Babka (SUSE)" Content-Language: en-US Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: CD7E140008 X-Stat-Signature: ar5j3ju1ohp4n8bw4yhwtxqb36aqei7i X-Spam-Status: No, score=-17.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-Rspam-User: X-Rspamd-Server: rspam09 X-HE-Tag: 1792480036-749163 X-HE-Meta: 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 500qwcpz 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 10/16/26 18:23, Rik van Riel wrote: > With debug_guardpage_minorder set, expand() turns the unused halves of a > split into guard pages, which page_del_and_expand() leaves out of the free > counts. Freeing the allocated half credits only that half, and > __free_one_page() then merges the guard buddy with clear_page_guard(), > which no longer touches the counters. > > The merged block goes on a free list with the guard pages never counted, > so NR_FREE_PAGES falls behind the free lists by every guard that merges. > Before commit e0932b6c1f94 ("mm: page_alloc: consolidate free page > accounting"), __set_page_guard() and __clear_page_guard() adjusted the > counts themselves; that commit dropped both adjustments but kept the > subtraction implicit in expand(). > > Count the guard pages under the merged block's migratetype when the > buddy is cleared. account_freepages() skips an isolated block as it does > for every other free page, and moving the block off the isolated list > counts it then. > > Booting a 16GB VM with debug_pagealloc=on debug_guardpage_minorder=1, > check the difference in free pages reported between /proc/vmstat > and /proc/buddyinfo. > > At three points (idle, after a read and file-creation load, after > a second load), check the difference between nr_free_pages and the > buddyinfo numbers in the normal zone, as a number of pages: > > idle after load after second load > unpatched -130 79205 99121 > patched -16 45 0 > > The small remaining difference seems to be due to the numbers > not being read at exactly the same time, and is also seen > without guard pages. > > Fixes: e0932b6c1f94 ("mm: page_alloc: consolidate free page accounting") > Cc: stable@vger.kernel.org > Assisted-by: LLM > Signed-off-by: Rik van Riel > Reviewed-by: Zi Yan > Reviewed-by: Johannes Weiner > Reviewed-by: Dev Jain Reviewed-by: Vlastimil Babka (SUSE) > --- > mm/page_alloc.c | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 12fac9084c483..4658af97be010 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -988,10 +988,12 @@ static inline void __free_one_page(struct page *page, > * Our buddy is free or it is CONFIG_DEBUG_PAGEALLOC guard page, > * merge with it and move up one order. > */ > - if (page_is_guard(buddy)) > + if (page_is_guard(buddy)) { > clear_page_guard(zone, buddy, order); > - else > + account_freepages(zone, 1 << order, migratetype); > + } else { > __del_page_from_free_list(buddy, zone, order, buddy_mt); > + } > > if (unlikely(buddy_mt != migratetype)) { > /* >