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Date: Mon, 13 May 2019 13:32:28 -0400 (EDT)
From: Pankaj Gupta <pagupta@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v8 3/6] libnvdimm: add dax_dev sync flag
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To: Dan Williams <dan.j.williams@intel.com>
Cc: cohuck@redhat.com, Jan Kara <jack@suse.cz>, KVM list <kvm@vger.kernel.org>, "Michael S. Tsirkin" <mst@redhat.com>, Jason Wang <jasowang@redhat.com>, david <david@fromorbit.com>, Qemu Developers <qemu-devel@nongnu.org>, virtualization@lists.linux-foundation.org, Andreas Dilger <adilger.kernel@dilger.ca>, Ross Zwisler <zwisler@kernel.org>, Andrea Arcangeli <aarcange@redhat.com>, Dave Jiang <dave.jiang@intel.com>, jstaron@google.com, linux-nvdimm <linux-nvdimm@lists.01.org>, Vishal L Verma <vishal.l.verma@intel.com>, David Hildenbrand <david@redhat.com>, Matthew Wilcox <willy@infradead.org>, Christoph Hellwig <hch@infradead.org>, Linux ACPI <linux-acpi@vger.kernel.org>, jmoyer <jmoyer@redhat.com>, linux-ext4 <linux-ext4@vger.kernel.org>, Len Brown <lenb@kernel.org>, Adam Borowski <kilobyte@angband.pl>, Rik van Riel <riel@surriel.com>, yuval shaia <yuval.shaia@oracle.com>, Stefan Hajnoczi <stefanha@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, lcapitulino@redhat.com, Kevin Wolf <kwolf@redhat.com>, Nitesh Narayan Lal <nilal@redhat.com>, Theodore Ts'o <tytso@mit.edu>, Xiao Guangrong <xiaoguangrong.eric@gmail.com>, "Darrick J. Wong" <darrick.wong@oracle.com>, "Rafael J. Wysocki" <rjw@rjwysocki.net>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-xfs <linux-xfs@vger.kernel.org>, linux-fsdevel <linux-fsdevel@vger.kernel.org>, Igor Mammedov <imammedo@redhat.com>


Hi Dan,

While testing device mapper with DAX, I faced a bug with the commit:

commit ad428cdb525a97d15c0349fdc80f3d58befb50df
Author: Dan Williams <dan.j.williams@intel.com>
Date:   Wed Feb 20 21:12:50 2019 -0800

When I reverted the condition to old code[1] it worked for me. I 
am thinking when we map two different devices (e.g with device mapper), will 
start & end pfn still point to same pgmap? Or there is something else which
I am missing here. 

Note: I tested only EXT4.

[1]

-               if (pgmap && pgmap->type == MEMORY_DEVICE_FS_DAX)
+               end_pgmap = get_dev_pagemap(pfn_t_to_pfn(end_pfn), NULL);
+               if (pgmap && pgmap == end_pgmap && pgmap->type == MEMORY_DEVICE_FS_DAX
+                               && pfn_t_to_page(pfn)->pgmap == pgmap
+                               && pfn_t_to_page(end_pfn)->pgmap == pgmap
+                               && pfn_t_to_pfn(pfn) == PHYS_PFN(__pa(kaddr))
+                               && pfn_t_to_pfn(end_pfn) == PHYS_PFN(__pa(end_kaddr)))
                        dax_enabled = true;
                put_dev_pagemap(pgmap);

Thanks,
Pankaj