Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753405AbdI0OBB (ORCPT ); Wed, 27 Sep 2017 10:01:01 -0400 Received: from mail-oi0-f54.google.com ([209.85.218.54]:54126 "EHLO mail-oi0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753057AbdI0OA5 (ORCPT ); Wed, 27 Sep 2017 10:00:57 -0400 X-Google-Smtp-Source: AOwi7QDV5R2Z06MEmEPVrT9798Y93RuHiKKH9tdtfVWbvKOFyE28FK+66BIqRhbaXnDoJez1JXGbWlEFUJIJ52r7tcw= MIME-Version: 1.0 In-Reply-To: <20170927113527.GD25746@quack2.suse.cz> References: <20170925231404.32723-1-ross.zwisler@linux.intel.com> <20170925231404.32723-7-ross.zwisler@linux.intel.com> <20170926185751.GB31146@linux.intel.com> <20170926210645.GA7798@linux.intel.com> <20170927113527.GD25746@quack2.suse.cz> From: Dan Williams Date: Wed, 27 Sep 2017 07:00:53 -0700 Message-ID: Subject: Re: [PATCH 6/7] mm, fs: introduce file_operations->post_mmap() To: Jan Kara Cc: Ross Zwisler , Andrew Morton , "linux-kernel@vger.kernel.org" , "Darrick J. Wong" , "J. Bruce Fields" , Christoph Hellwig , Dave Chinner , Jeff Layton , linux-fsdevel , Linux MM , "linux-nvdimm@lists.01.org" , linux-xfs@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2724 Lines: 50 On Wed, Sep 27, 2017 at 4:35 AM, Jan Kara wrote: > On Tue 26-09-17 14:41:53, Dan Williams wrote: >> On Tue, Sep 26, 2017 at 2:06 PM, Ross Zwisler >> wrote: >> > On Tue, Sep 26, 2017 at 12:19:21PM -0700, Dan Williams wrote: >> >> On Tue, Sep 26, 2017 at 11:57 AM, Ross Zwisler >> > <> >> >> > This decision can only be made (in this >> >> > proposed scheme) *after* the inode->i_mapping->i_mmap tree has been >> >> > populated, which means we need another call into the filesystem after this >> >> > insertion has happened. >> >> >> >> I get that, but it seems over-engineered and something that can also >> >> be safely cleaned up after the fact by the code path that is disabling >> >> DAX. >> > >> > I don't think you can safely clean it up after the fact because some thread >> > might have already called ->mmap() to set up the vma->vm_flags for their new >> > mapping, but they haven't added it to inode->i_mapping->i_mmap. >> >> If madvise(MADV_NOHUGEPAGE) can dynamically change vm_flags, then the >> DAX disable path can as well. VM_MIXEDMAP looks to be a nop for normal >> memory mappings. >> >> > The inode->i_mapping->i_mmap tree is the only way (that I know of at least) >> > that the filesystem has any idea about about the mapping. This is the method >> > by which we would try and clean up mapping flags, if we were to do so, and >> > it's the only way that the filesystem can know whether or not mappings exist. >> > >> > The only way that I could think of to make this safely work is to have the >> > insertion into the inode->i_mapping->i_mmap tree be our sync point. After >> > that the filesystem and the mapping code can communicate on the state of DAX, >> > but before that I think it's basically indeterminate. >> >> If we lose the race and leak VM_HUGEPAGE to a non-DAX mapping what >> breaks? I'd rather be in favor of not setting VM_HUGEPAGE at all in >> the ->mmap() handler and let the default THP policy take over. In >> fact, see transparent_hugepage_enabled() we already auto-enable huge >> page support for dax mappings regardless of VM_HUGEPAGE. > > Hum, this is an interesting option. So do you suggest that filesystems > supporting DAX would always setup mappings with VM_MIXEDMAP and without > VM_HUGEPAGE and thus we'd get rid of dependency on S_DAX flag in ->mmap? > That could actually work. The only possible issue I can see is that > VM_MIXEDMAP is still slightly different from normal page mappings and it > could have some performance implications - e.g. copy_page_range() does more > work on VM_MIXEDMAP mappings but not on normal page mappings. We can also get rid of VM_MIXEDMAP if we disable DAX in the !pfn_t_has_page() case.