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Wed, 14 Jul 2021 17:39:27 +0000 Subject: Re: [External] [PATCH 3/3] hugetlb: before freeing hugetlb page set dtor to appropriate value To: Muchun Song Cc: Linux Memory Management List , LKML , Michal Hocko , Oscar Salvador , David Hildenbrand , Matthew Wilcox , Naoya Horiguchi , Mina Almasry , Andrew Morton References: <20210710002441.167759-1-mike.kravetz@oracle.com> <20210710002441.167759-4-mike.kravetz@oracle.com> From: Mike Kravetz Message-ID: <6c38223b-83f4-ef7d-68d7-27c0f6ae6359@oracle.com> Date: Wed, 14 Jul 2021 10:39:25 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MW4P221CA0014.NAMP221.PROD.OUTLOOK.COM (2603:10b6:303:8b::19) To BY5PR10MB4196.namprd10.prod.outlook.com (2603:10b6:a03:20d::23) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [192.168.2.112] (50.38.35.18) by MW4P221CA0014.NAMP221.PROD.OUTLOOK.COM (2603:10b6:303:8b::19) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4331.21 via Frontend Transport; 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This is needed in case someone takes an additional >> + * temporary ref to the page, and freeing is delayed until they drop >> + * their reference. >> + * >> + * For gigantic pages set the destructor to the null dtor. This >> + * destructor will never be called. Before freeing the gigantic >> + * page destroy_compound_gigantic_page will turn the compound page >> + * into a simple group of pages. After this the destructor does not >> + * apply. >> + * >> + * This handles the case where more than one ref is held when and >> + * after update_and_free_page is called. >> + */ >> set_page_refcounted(page); >> - set_compound_page_dtor(page, NULL_COMPOUND_DTOR); >> + if (hstate_is_gigantic(h)) >> + set_compound_page_dtor(page, NULL_COMPOUND_DTOR); >> + else >> + set_compound_page_dtor(page, COMPOUND_PAGE_DTOR); > > Hi Mike, > > The race is really subtle. But we also should remove the WARN from > free_contig_range, right? Because the refcount of the head page of > the gigantic page can be greater than one, but free_contig_range has > the following warning. > > WARN(count != 0, "%lu pages are still in use!\n", count); > I did hit that warning in my testing and thought about removing it. However, I decided to keep it because non-hugetlb code also makes use of alloc_contig_range/free_contig_range and it might be useful in those cases. My 'guess' is that the warning was added not because of temporary ref count increases but rather to point out any code that forgot to drop a reference. BTW - It is not just the 'head' page which could trigger this warning, but any 'tail' page as well. That is because we do not call free_contig_range with a compound page, but rather a group of pages all with ref count of at least one. I'm happy to remove the warning if people do not think it is generally useful. -- Mike Kravetz