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Thu, 22 Oct 2020 02:33:57 +0000 Date: Wed, 21 Oct 2020 19:33:52 -0700 From: Roman Gushchin To: Mike Kravetz CC: Andrew Morton , Zi Yan , Joonsoo Kim , , , Subject: Re: [PATCH rfc 0/2] mm: cma: make cma_release() non-blocking Message-ID: <20201022023352.GC300658@carbon.dhcp.thefacebook.com> References: <20201016225254.3853109-1-guro@fb.com> <3f455d27-6d99-972f-b77f-b5b473b7614d@oracle.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3f455d27-6d99-972f-b77f-b5b473b7614d@oracle.com> X-Originating-IP: [2620:10d:c090:400::4:4b52] X-ClientProxiedBy: MWHPR10CA0010.namprd10.prod.outlook.com (2603:10b6:301::20) To BYAPR15MB4136.namprd15.prod.outlook.com (2603:10b6:a03:96::24) X-MS-Exchange-MessageSentRepresentingType: 1 Received: from carbon.dhcp.thefacebook.com (2620:10d:c090:400::4:4b52) by MWHPR10CA0010.namprd10.prod.outlook.com (2603:10b6:301::20) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3499.18 via Frontend Transport; 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X-MS-Exchange-AntiSpam-MessageData: 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 X-MS-Exchange-CrossTenant-Network-Message-Id: c8ee7e03-05f1-4091-45cc-08d87632ecd2 X-MS-Exchange-CrossTenant-AuthSource: BYAPR15MB4136.namprd15.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 22 Oct 2020 02:33:56.9209 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 8ae927fe-1255-47a7-a2af-5f3a069daaa2 X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: Uskc9dyQPcRgj2ReMEHJlubLK9QW/NrHcIZ4GU0BpJuTWngyvTPHyB38tJJrXYAZ X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR15MB2807 X-OriginatorOrg: fb.com X-Proofpoint-UnRewURL: 0 URL was un-rewritten MIME-Version: 1.0 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.235,18.0.737 definitions=2020-10-22_01:2020-10-20,2020-10-22 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 malwarescore=0 priorityscore=1501 bulkscore=0 lowpriorityscore=0 spamscore=0 clxscore=1015 suspectscore=0 adultscore=0 mlxlogscore=999 impostorscore=0 phishscore=0 mlxscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2010220015 X-FB-Internal: deliver 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 Wed, Oct 21, 2020 at 05:15:53PM -0700, Mike Kravetz wrote: > 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. Hi Mike, > > 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? I actually have suggested it few months ago, but the idea was opposed by Joonsoo: https://lkml.org/lkml/2020/4/3/12 . The time of a bitmap operation is definitely not an issue in my context, but I can't speak for something like an embedded/rt case. Thanks!