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Tue, 16 Sep 2025 05:50:55 GMT Received: from smtpav03.wdc07v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 3F4205805A; Tue, 16 Sep 2025 05:50:55 +0000 (GMT) Received: from smtpav03.wdc07v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id DB6D15805C; Tue, 16 Sep 2025 05:50:50 +0000 (GMT) Received: from [9.109.215.183] (unknown [9.109.215.183]) by smtpav03.wdc07v.mail.ibm.com (Postfix) with ESMTP; Tue, 16 Sep 2025 05:50:50 +0000 (GMT) Message-ID: <7dcfab94-753a-4e60-b350-2a5f09613c34@linux.ibm.com> Date: Tue, 16 Sep 2025 11:20:49 +0530 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/3] mm/ksm: Fix incorrect KSM counter handling in mm_struct during fork To: Sasha Levin , Andrew Morton Cc: David Hildenbrand , Ritesh Harjani , Xu Xin , Chengming Zhou , Wei Yang , Aboorva Devarajan , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Giorgi Tchankvetadze , stable@vger.kernel.org, Joe Perches References: <4044e7623953d9f4c240d0308cf0b2fe769ee553.1757946863.git.donettom@linux.ibm.com> <20250915164248.788601c4dc614913081ec7d7@linux-foundation.org> Content-Language: en-US From: Donet Tom In-Reply-To: Content-Type: text/plain; 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This results in >>> inconsistent >>> accounting. >>> >>> When a process uses KSM, identical pages are merged and an rmap item is >>> created for each merged page. The `ksm_merging_pages` and >>> `ksm_rmap_items` counters are updated accordingly. However, after a >>> fork, these counters are copied to the child while the corresponding >>> rmap items are not. As a result, when the child later triggers an >>> unmerge, there are no rmap items present in the child, so the counters >>> remain stale, leading to incorrect accounting. >>> >>> A similar issue exists with `ksm_zero_pages`, which maintains both a >>> global counter and a per-process counter. During fork, the per-process >>> counter is inherited by the child, but the global counter is not >>> incremented. Since the child also references zero pages, the global >>> counter should be updated as well. Otherwise, during zero-page unmerge, >>> both the global and per-process counters are decremented, causing the >>> global counter to become inconsistent. >>> >>> To fix this, ksm_merging_pages and ksm_rmap_items are reset to 0 >>> during fork, and the global ksm_zero_pages counter is updated with the >>> per-process ksm_zero_pages value inherited by the child. This ensures >>> that KSM statistics remain accurate and reflect the activity of each >>> process correctly. >>> >>> Fixes: 7609385337a4 ("ksm: count ksm merging pages for each process") >> >> Linux-v5.19 >> >>> Fixes: cb4df4cae4f2 ("ksm: count allocated ksm rmap_items for each >>> process") >> >> Linux-v6.1 >> >>> Fixes: e2942062e01d ("ksm: count all zero pages placed by KSM") >> >> Linux-v6.10 >> >>> cc: stable@vger.kernel.org # v6.6 >> >> So how was Linux-v6.6 arrived at? > > e2942062e01d is in v6.6, not in v6.10 - I suspect that this is why the > "# v6.6" > part was added. Yes, e2942062e01d is in v6.6, which is why I mentioned that we need to backport it up to v6.6. > >> I think the most important use for Fixes: is to tell the -stable >> maintainers which kernel version(s) we believe should receive the >> patch.  So listing multiple Fixes: targets just causes confusion. > > Right - there's no way of communicating if all the commits listed in > multiple > Fixes tags should exist in the tree, or any one of them, for the new > fix to be > applicable. >