From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751754AbdFUMcH (ORCPT ); Wed, 21 Jun 2017 08:32:07 -0400 Received: from foss.arm.com ([217.140.101.70]:52018 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751140AbdFUMcF (ORCPT ); Wed, 21 Jun 2017 08:32:05 -0400 From: Punit Agrawal To: Andrew Morton Cc: , , , , , , , , , , , Subject: Re: [PATCH v5 0/8] Support for contiguous pte hugepages References: <20170619170145.25577-1-punit.agrawal@arm.com> <20170619150133.cb4173220e4e3abd02c6f6d0@linux-foundation.org> <871sqezsk2.fsf@e105922-lin.cambridge.arm.com> <20170620140831.6bd835649d475bcf30c3c434@linux-foundation.org> Date: Wed, 21 Jun 2017 13:32:02 +0100 In-Reply-To: <20170620140831.6bd835649d475bcf30c3c434@linux-foundation.org> (Andrew Morton's message of "Tue, 20 Jun 2017 14:08:31 -0700") Message-ID: <87fuety119.fsf@e105922-lin.cambridge.arm.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Andrew Morton writes: > On Tue, 20 Jun 2017 14:39:57 +0100 Punit Agrawal wrote: > >> >> The architecture supports two flavours of hugepages - >> >> * Block mappings at the pud/pmd level >> >> These are regular hugepages where a pmd or a pud page table entry >> points to a block of memory. Depending on the PAGE_SIZE in use the >> following size of block mappings are supported - >> >> PMD PUD >> --- --- >> 4K: 2M 1G >> 16K: 32M >> 64K: 512M >> >> For certain applications/usecases such as HPC and large enterprise >> workloads, folks are using 64k page size but the minimum hugepage size >> of 512MB isn't very practical. >> >> To overcome this ... >> >> * Using the Contiguous bit >> >> The architecture provides a contiguous bit in the translation table >> entry which acts as a hint to the mmu to indicate that it is one of a >> contiguous set of entries that can be cached in a single TLB entry. >> >> We use the contiguous bit in Linux to increase the mapping size at the >> pmd and pte (last) level. >> >> The number of supported contiguous entries varies by page size and >> level of the page table. >> >> Using the contiguous bit allows additional hugepage sizes - >> >> CONT PTE PMD CONT PMD PUD >> -------- --- -------- --- >> 4K: 64K 2M 32M 1G >> 16K: 2M 32M 1G >> 64K: 2M 512M 16G >> >> Of these, 64K with 4K and 2M with 64K pages have been explicitly >> requested by a few different users. >> >> Entries with the contiguous bit set are required to be modified all >> together - which makes things like memory poisoning and migration >> impossible to do correctly without knowing the size of hugepage being >> dealt with - the reason for adding size parameter to a few of the >> hugepage helpers in this series. >> > > Thanks, I added the above to the 1/n changelog. Perhaps it's worth > adding something like this to Documentation/vm/hugetlbpage.txt. Yes, it would be useful to have this documented. I'll send a patch once the architecture bits for re-enabling contiguous hugepages are merged. Thanks, Punit