From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-yk0-f180.google.com (mail-yk0-f180.google.com [209.85.160.180]) by kanga.kvack.org (Postfix) with ESMTP id C44ED6B0038 for ; Wed, 2 Dec 2015 13:06:37 -0500 (EST) Received: by ykba77 with SMTP id a77so56194673ykb.2 for ; Wed, 02 Dec 2015 10:06:37 -0800 (PST) Received: from mail-yk0-x235.google.com (mail-yk0-x235.google.com. [2607:f8b0:4002:c07::235]) by mx.google.com with ESMTPS id j123si2706252ywe.31.2015.12.02.10.06.36 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 02 Dec 2015 10:06:36 -0800 (PST) Received: by ykdr82 with SMTP id r82so56164625ykd.3 for ; Wed, 02 Dec 2015 10:06:36 -0800 (PST) MIME-Version: 1.0 In-Reply-To: References: <1448309082-20851-1-git-send-email-toshi.kani@hpe.com> <1449022764.31589.24.camel@hpe.com> <1449078237.31589.30.camel@hpe.com> Date: Wed, 2 Dec 2015 10:06:36 -0800 Message-ID: Subject: Re: [PATCH] mm: Fix mmap MAP_POPULATE for DAX pmd mapping From: Dan Williams Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: To: Toshi Kani Cc: Andrew Morton , "Kirill A. Shutemov" , Matthew Wilcox , Ross Zwisler , mauricio.porto@hpe.com, Linux MM , linux-fsdevel , "linux-nvdimm@lists.01.org" , "linux-kernel@vger.kernel.org" On Wed, Dec 2, 2015 at 9:01 AM, Dan Williams wrote: > On Wed, Dec 2, 2015 at 9:43 AM, Toshi Kani wrote: >> Oh, I see. I will setup the memmap array and run the tests again. >> >> But, why does the PMD mapping depend on the memmap array? We have observed >> major performance improvement with PMD. This feature should always be enabled >> with DAX regardless of the option to allocate the memmap array. >> > > Several factors drove this decision, I'm open to considering > alternatives but here's the reasoning: > > 1/ DAX pmd mappings caused crashes in the get_user_pages path leading > to commit e82c9ed41e8 "dax: disable pmd mappings". The reason pte > mappings don't crash and instead trigger -EFAULT is due to the > _PAGE_SPECIAL pte bit. > > 2/ To enable get_user_pages for DAX, in both the page and huge-page > case, we need a new pte bit _PAGE_DEVMAP. > > 3/ Given the pte bits are hard to come I'm assuming we won't get two, > i.e. both _PAGE_DEVMAP and a new _PAGE_SPECIAL for pmds. Even if we > could get a _PAGE_SPECIAL for pmds I'm not in favor of pursuing it. Actually, Dave says they aren't that hard to come by for pmds, so we could go add _PMD_SPECIAL if we really wanted to support the limited page-less DAX-pmd case. But I'm still of the opinion that we run away from the page-less case until it can be made a full class citizen with O_DIRECT for pfn support. -- 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