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Tue, 04 Feb 2020 21:42:47 +0000 Received: from pps.filterd (userp3020.oracle.com [127.0.0.1]) by userp3020.oracle.com (8.16.0.27/8.16.0.27) with SMTP id 014LdUtE119922; Tue, 4 Feb 2020 21:42:46 GMT Received: from userv0121.oracle.com (userv0121.oracle.com [156.151.31.72]) by userp3020.oracle.com with ESMTP id 2xxvy3yh04-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 04 Feb 2020 21:42:46 +0000 Received: from abhmp0015.oracle.com (abhmp0015.oracle.com [141.146.116.21]) by userv0121.oracle.com (8.14.4/8.13.8) with ESMTP id 014LgiQP028259; Tue, 4 Feb 2020 21:42:45 GMT Received: from [192.168.1.206] (/71.63.128.209) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Tue, 04 Feb 2020 13:42:44 -0800 Subject: Re: [PATCH] mm: always consider THP when adjusting min_free_kbytes To: David Rientjes Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Song Liu , "Kirill A.Shutemov" , Mel Gorman , Vlastimil Babka , Andrew Morton References: <20200204194156.61672-1-mike.kravetz@oracle.com> From: Mike Kravetz Message-ID: <8cc18928-0b52-7c2e-fbc6-5952eb9b06ab@oracle.com> Date: Tue, 4 Feb 2020 13:42:43 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9521 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=2 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1911140001 definitions=main-2002040148 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9521 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 priorityscore=1501 malwarescore=0 suspectscore=2 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1911140001 definitions=main-2002040148 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 2/4/20 12:33 PM, David Rientjes wrote: > On Tue, 4 Feb 2020, Mike Kravetz wrote: > > Hmm, if khugepaged_adjust_min_free_kbytes() increases min_free_kbytes for > thp, then the user has no ability to override this increase by using > vm.min_free_kbytes? > > IIUC, with this change, it looks like memory hotplug events properly > increase min_free_kbytes for thp optimization but also doesn't respect a > previous user-defined value? Good catch. We should only call khugepaged_adjust_min_free_kbytes from the 'true' block of this if statement in init_per_zone_wmark_min. if (new_min_free_kbytes > user_min_free_kbytes) { min_free_kbytes = new_min_free_kbytes; if (min_free_kbytes < 128) min_free_kbytes = 128; if (min_free_kbytes > 65536) min_free_kbytes = 65536; } else { pr_warn("min_free_kbytes is not updated to %d because user defined value %d is preferred\n", new_min_free_kbytes, user_min_free_kbytes); } In the existing code, a hotplug event will cause min_free_kbytes to overwrite the user defined value if the new value is greater. However, you will get the warning message if the user defined value is greater. I am not sure if this is the 'desired/expected' behavior? We print a warning if the user value takes precedence over our calculated value. However, we do not print a message if we overwrite the user defined value. That doesn't seem right! > So it looks like this is fixing an obvious correctness issue but also now > requires users to rewrite the sysctl if they want to decrease the min > watermark. Moving the call to khugepaged_adjust_min_free_kbytes as described above would avoid the THP adjustment unless we were going to overwrite the user defined value. Now, I am not sure overwriting the user defined value as is done today is actually the correct thing to do. Thoughts? Perhaps we should never overwrite a user defined value? -- Mike Kravetz