From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 163D522F74F; Tue, 16 Sep 2025 04:41:18 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757997679; cv=none; b=FW9egxaW3Zk5o8rFfRCL6nglQHHRQzhl89ylU4dbEWukxg2NWefG/TKH8L/7a5M1yDkiMioaNrFK2aze+nBQaBCOrxJXW5COwBj38U/RHdInTarfBJDZU2LX+cfqgLIHG7n4MkIVbMDzmI2qY2RA1LFHl8WzgpMmiUg98DtCMmc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757997679; c=relaxed/simple; bh=sNER0piXqThOPvVZ2lFH5TPIw7AbRv56V9c6OrCJQ9A=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=rNz+6fg7Qu+YdRuvyTzQEeKeNyLBsojSyHZgzhXnZndGZM0G2QxaUThmGqnyZY8Ozca5j00aW7vZaxALUdrxVY3Zx59bd10u05Y8r0CLgJMgL59askgQ6XtvAWgmggfNrvBWFKkGcSg74GlQpfkqh+EJblzc1NeMGqN8oqXLJI0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=kfde2bCK; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="kfde2bCK" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 37F91C4CEEB; Tue, 16 Sep 2025 04:41:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1757997678; bh=sNER0piXqThOPvVZ2lFH5TPIw7AbRv56V9c6OrCJQ9A=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=kfde2bCK6bSYDIyOsrhYTuiq9ts/r+Q8UslL3g+/PNOzNfNI9QUaZ2gNWFz8JRet0 saavCH1g6XV2gUOYRLpDvPtr1OvySz2puf6DKyAIeu10hnBDjiHtAc6mOGUCtra3m7 zNoUgcL3ii6ytSLE8fbvUvOyjN8FxGCBcTMqjPq4= Date: Mon, 15 Sep 2025 21:41:17 -0700 From: Andrew Morton To: Sasha Levin Cc: Donet Tom , 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 Subject: Re: [PATCH v2 1/3] mm/ksm: Fix incorrect KSM counter handling in mm_struct during fork Message-Id: <20250915214117.5117d339669e091b1d3fa96d@linux-foundation.org> In-Reply-To: References: <4044e7623953d9f4c240d0308cf0b2fe769ee553.1757946863.git.donettom@linux.ibm.com> <20250915164248.788601c4dc614913081ec7d7@linux-foundation.org> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Tue, 16 Sep 2025 00:33:09 -0400 Sasha Levin wrote: > On Mon, Sep 15, 2025 at 04:42:48PM -0700, Andrew Morton wrote: > >On Mon, 15 Sep 2025 20:33:04 +0530 Donet Tom wrote: > > > >> Currently, the KSM-related counters in `mm_struct`, such as > >> `ksm_merging_pages`, `ksm_rmap_items`, and `ksm_zero_pages`, are > >> inherited by the child process during fork. 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. OK. > > >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. So what should we do in this situation?