From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965558AbdEWNUm (ORCPT ); Tue, 23 May 2017 09:20:42 -0400 Received: from mail-wm0-f67.google.com ([74.125.82.67]:35958 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934140AbdEWNUR (ORCPT ); Tue, 23 May 2017 09:20:17 -0400 Date: Tue, 23 May 2017 16:13:12 +0300 From: "Kirill A. Shutemov" To: Punit Agrawal Cc: akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com, will.deacon@arm.com, n-horiguchi@ah.jp.nec.com, kirill.shutemov@linux.intel.com, mike.kravetz@oracle.com, steve.capper@arm.com, mark.rutland@arm.com, hillf.zj@alibaba-inc.com, linux-arch@vger.kernel.org, aneesh.kumar@linux.vnet.ibm.com, Michal Hocko Subject: Re: [PATCH v3 2/6] mm, gup: Ensure real head page is ref-counted when using hugepages Message-ID: <20170523131312.aim6obne2t5sxtdr@node.shutemov.name> References: <20170522133604.11392-1-punit.agrawal@arm.com> <20170522133604.11392-3-punit.agrawal@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170522133604.11392-3-punit.agrawal@arm.com> User-Agent: NeoMutt/20170428 (1.8.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 22, 2017 at 02:36:00PM +0100, Punit Agrawal wrote: > When speculatively taking references to a hugepage using > page_cache_add_speculative() in gup_huge_pmd(), it is assumed that the > page returned by pmd_page() is the head page. Although normally true, > this assumption doesn't hold when the hugepage comprises of successive > page table entries such as when using contiguous bit on arm64 at PTE or > PMD levels. > > This can be addressed by ensuring that the page passed to > page_cache_add_speculative() is the real head or by de-referencing the > head page within the function. > > We take the first approach to keep the usage pattern aligned with > page_cache_get_speculative() where users already pass the appropriate > page, i.e., the de-referenced head. > > Apply the same logic to fix gup_huge_[pud|pgd]() as well. Hm. Okay. But I'm kinda surprise that this is the only place that need to be adjusted. Have you validated all other pmd_page() use-cases? -- Kirill A. Shutemov From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Kirill A. Shutemov" Subject: Re: [PATCH v3 2/6] mm, gup: Ensure real head page is ref-counted when using hugepages Date: Tue, 23 May 2017 16:13:12 +0300 Message-ID: <20170523131312.aim6obne2t5sxtdr@node.shutemov.name> References: <20170522133604.11392-1-punit.agrawal@arm.com> <20170522133604.11392-3-punit.agrawal@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20170522133604.11392-3-punit.agrawal@arm.com> Sender: owner-linux-mm@kvack.org To: Punit Agrawal Cc: akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com, will.deacon@arm.com, n-horiguchi@ah.jp.nec.com, kirill.shutemov@linux.intel.com, mike.kravetz@oracle.com, steve.capper@arm.com, mark.rutland@arm.com, hillf.zj@alibaba-inc.com, linux-arch@vger.kernel.org, aneesh.kumar@linux.vnet.ibm.com, Michal Hocko List-Id: linux-arch.vger.kernel.org On Mon, May 22, 2017 at 02:36:00PM +0100, Punit Agrawal wrote: > When speculatively taking references to a hugepage using > page_cache_add_speculative() in gup_huge_pmd(), it is assumed that the > page returned by pmd_page() is the head page. Although normally true, > this assumption doesn't hold when the hugepage comprises of successive > page table entries such as when using contiguous bit on arm64 at PTE or > PMD levels. > > This can be addressed by ensuring that the page passed to > page_cache_add_speculative() is the real head or by de-referencing the > head page within the function. > > We take the first approach to keep the usage pattern aligned with > page_cache_get_speculative() where users already pass the appropriate > page, i.e., the de-referenced head. > > Apply the same logic to fix gup_huge_[pud|pgd]() as well. Hm. Okay. But I'm kinda surprise that this is the only place that need to be adjusted. Have you validated all other pmd_page() use-cases? -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org From mboxrd@z Thu Jan 1 00:00:00 1970 From: kirill@shutemov.name (Kirill A. Shutemov) Date: Tue, 23 May 2017 16:13:12 +0300 Subject: [PATCH v3 2/6] mm, gup: Ensure real head page is ref-counted when using hugepages In-Reply-To: <20170522133604.11392-3-punit.agrawal@arm.com> References: <20170522133604.11392-1-punit.agrawal@arm.com> <20170522133604.11392-3-punit.agrawal@arm.com> Message-ID: <20170523131312.aim6obne2t5sxtdr@node.shutemov.name> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Mon, May 22, 2017 at 02:36:00PM +0100, Punit Agrawal wrote: > When speculatively taking references to a hugepage using > page_cache_add_speculative() in gup_huge_pmd(), it is assumed that the > page returned by pmd_page() is the head page. Although normally true, > this assumption doesn't hold when the hugepage comprises of successive > page table entries such as when using contiguous bit on arm64 at PTE or > PMD levels. > > This can be addressed by ensuring that the page passed to > page_cache_add_speculative() is the real head or by de-referencing the > head page within the function. > > We take the first approach to keep the usage pattern aligned with > page_cache_get_speculative() where users already pass the appropriate > page, i.e., the de-referenced head. > > Apply the same logic to fix gup_huge_[pud|pgd]() as well. Hm. Okay. But I'm kinda surprise that this is the only place that need to be adjusted. Have you validated all other pmd_page() use-cases? -- Kirill A. Shutemov