linux-kernel - Re: [Qemu-devel] [PATCH v8 3/6] libnvdimm: add dax

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Date:   Mon, 13 May 2019 10:52:39 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Pankaj Gupta <pagupta@...hat.com>
Cc:     cohuck@...hat.com, Jan Kara <jack@...e.cz>,
        KVM list <kvm@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>, david <david@...morbit.com>,
        Qemu Developers <qemu-devel@...gnu.org>,
        virtualization@...ts.linux-foundation.org,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Ross Zwisler <zwisler@...nel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Dave Jiang <dave.jiang@...el.com>, jstaron@...gle.com,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@...radead.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        jmoyer <jmoyer@...hat.com>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        Len Brown <lenb@...nel.org>,
        Adam Borowski <kilobyte@...band.pl>,
        Rik van Riel <riel@...riel.com>,
        yuval shaia <yuval.shaia@...cle.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>, lcapitulino@...hat.com,
        Kevin Wolf <kwolf@...hat.com>,
        Nitesh Narayan Lal <nilal@...hat.com>,
        "Theodore Ts'o" <tytso@....edu>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Igor Mammedov <imammedo@...hat.com>
Subject: Re: [Qemu-devel] [PATCH v8 3/6] libnvdimm: add dax_dev sync flag

On Mon, May 13, 2019 at 10:32 AM Pankaj Gupta <pagupta@...hat.com> wrote:
>
>
> Hi Dan,
>
> While testing device mapper with DAX, I faced a bug with the commit:
>
> commit ad428cdb525a97d15c0349fdc80f3d58befb50df
> Author: Dan Williams <dan.j.williams@...el.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)))

Ugh, yes, device-mapper continues to be an awkward fit for dax (or
vice versa). We would either need a way to have a multi-level pfn to
pagemap lookup for composite devices, or a way to discern that even
though the pagemap is different that the result is still valid / not
an indication that we have leaked into an unassociated address range.
Perhaps a per-daxdev callback for ->dax_supported() so that
device-mapper internals can be used for this validation.

We need to get that fixed up, but I don't see it as a blocker /
pre-requisite for virtio-pmem.