From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: MIME-Version: 1.0 In-Reply-To: <1449078237.31589.30.camel@hpe.com> 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 09:01:36 -0800 Message-ID: Subject: Re: [PATCH] mm: Fix mmap MAP_POPULATE for DAX pmd mapping From: Dan Williams 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" Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: 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. End result is that DAX pmd mappings must be fully enabled through the get_user_pages paths with _PAGE_DEVMAP or turned off completely. In general I think the "page less" DAX implementation was a good starting point, but we need to shift to page-backed by default until we can teach more of the kernel to operate on bare pfns. That "default" will need to be enforced by userspace tooling. -- 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