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Wed, 27 Aug 2025 18:03:10 GMT Received: from smtpav06.dal12v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id E28EB5805D; Wed, 27 Aug 2025 18:03:09 +0000 (GMT) Received: from smtpav06.dal12v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id E545658043; Wed, 27 Aug 2025 18:03:06 +0000 (GMT) Received: from [9.124.214.234] (unknown [9.124.214.234]) by smtpav06.dal12v.mail.ibm.com (Postfix) with ESMTP; Wed, 27 Aug 2025 18:03:06 +0000 (GMT) Message-ID: Date: Wed, 27 Aug 2025 23:33:05 +0530 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 1/2] mm/ksm: Reset KSM counters in mm_struct during fork To: David Hildenbrand , Andrew Morton Cc: Ritesh Harjani , Xu Xin , Chengming Zhou , Wei Yang , Aboorva Devarajan , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <2e662107e01417bf9af23bc7f52863cd538419be.1756211338.git.donettom@linux.ibm.com> <59fef25c-6559-4fb3-8810-c2a7dd36b95a@redhat.com> Content-Language: en-US From: Donet Tom In-Reply-To: <59fef25c-6559-4fb3-8810-c2a7dd36b95a@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: 8Fa1NHI_Oki9TJYL05TZ0jLzTna4d_S5 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwODIzMDAyMSBTYWx0ZWRfX1oh/PFuI1Lqq pZQ3mrxntSRXmSLKA7TeHRFQ2N6v0l4pvmouXrf1QyhmmorclGj+8hZ3JcoqC3FGuU9bSxcKePW fYwuGN57JIq0MIlVVemLGs9HTjeCIGClRXBZ3gP3+qUFME7RXmDh82Ci94xGZ/aDuHQO5d/1XJK nOWvG83Ehyt5Os2IF2l5tRew+UJINdx/5rHiIvxi5U3QkrndMcLy42IewIk9x7nAUXAEwwJdESy VKTDf44WukR34h8CUfAfaQ/JR1UynsVKaZBU7LcEuU7shOy9fA2QSB5IT0FN6vddj+oAdAk8ToZ UJbaIR/fzVsG2QPhMF8ahXfU3V/h8hQbShWhzLXY1wIKkSst72xQgy1rCRt/HS8IHalFFur2OOc iIGXRj7N X-Proofpoint-ORIG-GUID: iUe8y2EGS-BpnmODYpNQsowK0dTTA5XC X-Authority-Analysis: v=2.4 cv=SNNCVPvH c=1 sm=1 tr=0 ts=68af4860 cx=c_pps a=aDMHemPKRhS1OARIsFnwRA==:117 a=aDMHemPKRhS1OARIsFnwRA==:17 a=IkcTkHD0fZMA:10 a=2OwXVqhp2XgA:10 a=VnNF1IyMAAAA:8 a=RZE5Jt-HtoTff_lTyEUA:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1099,Hydra:6.1.9,FMLib:17.12.80.40 definitions=2025-08-27_04,2025-08-26_01,2025-03-28_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 adultscore=0 bulkscore=0 phishscore=0 clxscore=1015 impostorscore=0 malwarescore=0 suspectscore=0 priorityscore=1501 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2507300000 definitions=main-2508230021 On 8/26/25 6:49 PM, David Hildenbrand wrote: > On 26.08.25 14:49, 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 >> incorrect accounting, since the child has not performed any >> KSM page merging. > > So, the situation is that our child process maps these pages, but it > does not have any stable rmap items corresponding to these pages. Yes > > rmap_walk_ksm() spells that case out. > > Can you clarify that in the description here, and how both stats > correspond to rmap items? Sure I will add it in next version. > > What is the effective result of this misacounting? I assume only a > higher number than expected. > Yes the number will be higher than expected. > > >> >> To fix this, reset these counters to 0 in the newly created >> `mm_struct` during fork. This ensures that KSM statistics >> remain accurate and only reflect the activity of each process. >> > > Fixes? CC stable? I will add it in next version. > >> Signed-off-by: Donet Tom >> --- >>   include/linux/ksm.h | 6 +++++- >>   1 file changed, 5 insertions(+), 1 deletion(-) >> >> diff --git a/include/linux/ksm.h b/include/linux/ksm.h >> index 22e67ca7cba3..61b8892c632b 100644 >> --- a/include/linux/ksm.h >> +++ b/include/linux/ksm.h >> @@ -56,8 +56,12 @@ static inline long mm_ksm_zero_pages(struct >> mm_struct *mm) >>   static inline void ksm_fork(struct mm_struct *mm, struct mm_struct >> *oldmm) >>   { >>       /* Adding mm to ksm is best effort on fork. */ >> -    if (mm_flags_test(MMF_VM_MERGEABLE, oldmm)) >> +    if (mm_flags_test(MMF_VM_MERGEABLE, oldmm)) { >> +        mm->ksm_merging_pages = 0; >> +        mm->ksm_rmap_items = 0; >> +        atomic_long_set(&mm->ksm_zero_pages, 0); >>           __ksm_enter(mm); >> +    } >>   } >>     static inline int ksm_execve(struct mm_struct *mm) > >