linux-ext4 - Re: [PATCH v8 3/6] libnvdimm: add dax

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Date:   Fri, 10 May 2019 13:14:07 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Pankaj Gupta <pagupta@...hat.com>
Cc:     linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        KVM list <kvm@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        Qemu Developers <qemu-devel@...gnu.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        Ross Zwisler <zwisler@...nel.org>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Christoph Hellwig <hch@...radead.org>,
        Len Brown <lenb@...nel.org>, Jan Kara <jack@...e.cz>,
        "Theodore Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        lcapitulino@...hat.com, Kevin Wolf <kwolf@...hat.com>,
        Igor Mammedov <imammedo@...hat.com>,
        jmoyer <jmoyer@...hat.com>,
        Nitesh Narayan Lal <nilal@...hat.com>,
        Rik van Riel <riel@...riel.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        david <david@...morbit.com>, cohuck@...hat.com,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Adam Borowski <kilobyte@...band.pl>,
        yuval shaia <yuval.shaia@...cle.com>, jstaron@...gle.com
Subject: Re: [PATCH v8 3/6] libnvdimm: add dax_dev sync flag

On Fri, May 10, 2019 at 8:53 AM Pankaj Gupta <pagupta@...hat.com> wrote:
>
> This patch adds 'DAXDEV_SYNC' flag which is set
> for nd_region doing synchronous flush. This later
> is used to disable MAP_SYNC functionality for
> ext4 & xfs filesystem for devices don't support
> synchronous flush.
>
> Signed-off-by: Pankaj Gupta <pagupta@...hat.com>
> ---
>  drivers/dax/bus.c            |  2 +-
>  drivers/dax/super.c          | 13 ++++++++++++-
>  drivers/md/dm.c              |  3 ++-
>  drivers/nvdimm/pmem.c        |  5 ++++-
>  drivers/nvdimm/region_devs.c |  7 +++++++
>  include/linux/dax.h          |  8 ++++++--
>  include/linux/libnvdimm.h    |  1 +
>  7 files changed, 33 insertions(+), 6 deletions(-)
[..]
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index 043f0761e4a0..ee007b75d9fd 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -1969,7 +1969,8 @@ static struct mapped_device *alloc_dev(int minor)
>         sprintf(md->disk->disk_name, "dm-%d", minor);
>
>         if (IS_ENABLED(CONFIG_DAX_DRIVER)) {
> -               dax_dev = alloc_dax(md, md->disk->disk_name, &dm_dax_ops);
> +               dax_dev = alloc_dax(md, md->disk->disk_name, &dm_dax_ops,
> +                                                        DAXDEV_F_SYNC);

Apologies for not realizing this until now, but this is broken.
Imaging a device-mapper configuration composed of both 'async'
virtio-pmem and 'sync' pmem. The 'sync' flag needs to be unified
across all members. I would change this argument to '0' and then
arrange for it to be set at dm_table_supports_dax() time after
validating that all components support synchronous dax.