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Wed, 27 Aug 2025 18:09:18 GMT Received: from smtpav06.dal12v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 9277958055; Wed, 27 Aug 2025 18:09:18 +0000 (GMT) Received: from smtpav06.dal12v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 49F8758043; Wed, 27 Aug 2025 18:09:15 +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 18:09:14 +0000 (GMT) Message-ID: Date: Wed, 27 Aug 2025 23:39:13 +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 , Giorgi Tchankvetadze Cc: aboorvad@linux.ibm.com, akpm@linux-foundation.org, chengming.zhou@linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org, richard.weiyang@gmail.com, ritesh.list@gmail.com, xu.xin16@zte.com.cn References: <2e662107e01417bf9af23bc7f52863cd538419be.1756211338.git.donettom@linux.ibm.com> <512764a4-611c-42d4-8b4a-2aaca4e519a4@gmail.com> <53a7bf3d-843d-4e19-b0a3-cc6852fe72c1@redhat.com> Content-Language: en-US From: Donet Tom In-Reply-To: <53a7bf3d-843d-4e19-b0a3-cc6852fe72c1@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: mS-jT1OluJPLxq8noMhODUtObNItOHqs X-Proofpoint-ORIG-GUID: W_QujhfOUYSUVaoozQfOXykh24azsJoS X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwODIzMDAyMSBTYWx0ZWRfX+NdWAw+hGmuK bVsLNdGQeZLHk3q4tmYipIsA/dlu0JGkf7ViJ3+k15NZckt954S2qyhu5FV6DI1hteHgrsUdG73 BUINunicVcSn0LEKstYS0y8JuDsY+up/ZaxD4jtUVSJQbW6+DhVHvyst+6F1izjdYzs5qk60lpa xDlpQ6BZymTbXdJlfzAFDuK/Bb/7Yxpssfb1cG+6KBFh7G8jSVJ+ItrsLT/Q52VXWLyZc/zxkxV Eqq++paV8dIaMF/DIQ7f0jdpg+9Ju++YHuX2fHLzNpNX4zwdYISUlsfwAYJFF5iHY4BCEil7M3J MyYG7p6LVAWUfo6/Q31YJChJliIfgXamffu058dZVaKCdWs2OwfQAKvzmf5wKoJpaa+1UAaZyrT YuIXP/AX X-Authority-Analysis: v=2.4 cv=A8ZsP7WG c=1 sm=1 tr=0 ts=68af49d1 cx=c_pps a=AfN7/Ok6k8XGzOShvHwTGQ==:117 a=AfN7/Ok6k8XGzOShvHwTGQ==:17 a=IkcTkHD0fZMA:10 a=2OwXVqhp2XgA:10 a=nXaOtthc6F8xOgzlwIkA: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 phishscore=0 adultscore=0 priorityscore=1501 suspectscore=0 clxscore=1015 spamscore=0 impostorscore=0 malwarescore=0 bulkscore=0 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 7:39 PM, David Hildenbrand wrote: > On 26.08.25 15:47, Giorgi Tchankvetadze wrote: >> What if we only allocate KSM stats when a process actually uses KSM? >> >> struct ksm_mm_stats { >>     atomic_long_t merging_pages; >>     atomic_long_t rmap_items; >>     atomic_long_t zero_pages; >> }; >> struct ksm_mm_stats *mm->ksm_stats; // NULL until first enter >> >> static inline struct ksm_mm_stats *mm_get_ksm_stats(struct mm_struct >> *mm) >> { >>     if (likely(mm->ksm_stats)) >>         return mm->ksm_stats; >>     return ksm_alloc_stats_if_needed(mm); // Slow path >> } > > The fork'ed child uses KSM. It just doesn't have any stable rmap entries. > > We have to copy the zero_pages counter such that > ksm_might_unmap_zero_page() will do the right thing. > > But you're comment made me realize that there is likely another bug: > > When copying zero_pages during fork(), we have to increment > &ksm_zero_pages as well. Otherwise we will get an underflow later. Yes, David, you are right. I added a test to check this scenario, and I am seeing ksm_zero_pages go negative. # cat /sys/kernel/mm/ksm/ksm_zero_pages -128 # > > @Donet, can you look into that as well? Sure, I will add a fix for this issue in the next version.