From mboxrd@z Thu Jan 1 00:00:00 1970 From: Punit Agrawal Subject: Re: [PATCH v3 2/6] mm, gup: Ensure real head page is ref-counted when using hugepages Date: Tue, 23 May 2017 16:43:50 +0100 Message-ID: <874lwbh93d.fsf@e105922-lin.cambridge.arm.com> References: <20170522133604.11392-1-punit.agrawal@arm.com> <20170522133604.11392-3-punit.agrawal@arm.com> <20170523131312.aim6obne2t5sxtdr@node.shutemov.name> Mime-Version: 1.0 Content-Type: text/plain Return-path: Received: from foss.arm.com ([217.140.101.70]:53804 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758470AbdEWPnw (ORCPT ); Tue, 23 May 2017 11:43:52 -0400 In-Reply-To: <20170523131312.aim6obne2t5sxtdr@node.shutemov.name> (Kirill A. Shutemov's message of "Tue, 23 May 2017 16:13:12 +0300") Sender: linux-arch-owner@vger.kernel.org List-ID: To: "Kirill A. Shutemov" 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 "Kirill A. Shutemov" writes: > 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? I came across the gup issues were found while investigating a failing test from mce-tests. I think the problem here is not due to the use of pmd_page() but because page_cache_[add|get]_speculative() don't ensure they ref-count the head page as is done in get_page(). Having said that, I had a quick look at the other uses of pmd_page() - Quite a few of them are followed by an explicit BUG_ON() to check that the page returned is a head page. All other instances seem to be dealing with transparent hugepages where contiguous hugepages are not supported. I don't see any call sites that ring alarm bells. Did you have any particular part of the code in mind where pmd_page() usage might be a problem? Thanks, Punit