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Thu, 22 Oct 2020 00:16:06 +0000 Received: from pps.filterd (aserp3030.oracle.com [127.0.0.1]) by aserp3030.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 09M0G6LN151197; Thu, 22 Oct 2020 00:16:06 GMT Received: from aserv0122.oracle.com (aserv0122.oracle.com [141.146.126.236]) by aserp3030.oracle.com with ESMTP id 348a6q0t8w-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 22 Oct 2020 00:16:06 +0000 Received: from abhmp0014.oracle.com (abhmp0014.oracle.com [141.146.116.20]) by aserv0122.oracle.com (8.14.4/8.14.4) with ESMTP id 09M0FsFh020722; Thu, 22 Oct 2020 00:15:55 GMT Received: from [192.168.2.112] (/50.38.35.18) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Wed, 21 Oct 2020 17:15:54 -0700 Subject: Re: [PATCH rfc 0/2] mm: cma: make cma_release() non-blocking To: Roman Gushchin , Andrew Morton Cc: Zi Yan , Joonsoo Kim , linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel-team@fb.com References: <20201016225254.3853109-1-guro@fb.com> From: Mike Kravetz Message-ID: <3f455d27-6d99-972f-b77f-b5b473b7614d@oracle.com> Date: Wed, 21 Oct 2020 17:15:53 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 In-Reply-To: <20201016225254.3853109-1-guro@fb.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9781 signatures=668682 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 malwarescore=0 mlxlogscore=999 bulkscore=0 spamscore=0 adultscore=0 suspectscore=0 mlxscore=0 phishscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2010220000 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9781 signatures=668682 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 phishscore=0 lowpriorityscore=0 priorityscore=1501 impostorscore=0 adultscore=0 bulkscore=0 malwarescore=0 mlxlogscore=999 mlxscore=0 spamscore=0 suspectscore=0 clxscore=1011 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2010220000 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 10/16/20 3:52 PM, Roman Gushchin wrote: > This small patchset makes cma_release() non-blocking and simplifies > the code in hugetlbfs, where previously we had to temporarily drop > hugetlb_lock around the cma_release() call. > > It should help Zi Yan on his work on 1 GB THPs: splitting a gigantic > THP under a memory pressure requires a cma_release() call. If it's > a blocking function, it complicates the already complicated code. > Because there are at least two use cases like this (hugetlbfs is > another example), I believe it's just better to make cma_release() > non-blocking. > > It also makes it more consistent with other memory releasing functions > in the kernel: most of them are non-blocking. Thanks for looking into this Roman. I may be missing something, but why does cma_release have to be blocking today? Certainly, it takes the bitmap in cma_clear_bitmap and could block. However, I do not see why cma->lock has to be a mutex. I may be missing something, but I do not see any code protected by the mutex doing anything that could sleep? Could we simply change that mutex to a spinlock? -- Mike Kravetz