linux-kernel - Re: [PATCH 0/3] dax: clear poison on the fly along pwrite

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Message-ID: <CAPcyv4hJZpoJPF5_6F+wUUjVY5fHXbVRsLfVNPSZxiS+7yzV0Q@mail.gmail.com>
Date:   Fri, 17 Sep 2021 12:37:51 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     "Darrick J. Wong" <djwong@...nel.org>,
        Jane Chu <jane.chu@...cle.com>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        "Weiny, Ira" <ira.weiny@...el.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Linux NVDIMM <nvdimm@...ts.linux.dev>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Shiyang Ruan <ruansy.fnst@...itsu.com>
Subject: Re: [PATCH 0/3] dax: clear poison on the fly along pwrite

On Fri, Sep 17, 2021 at 5:57 AM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Thu, Sep 16, 2021 at 11:40:28AM -0700, Dan Williams wrote:
> > > That was my gut feeling.  If everyone feels 100% comfortable with
> > > zeroingas the mechanism to clear poisoning I'll cave in.  The most
> > > important bit is that we do that through a dedicated DAX path instead
> > > of abusing the block layer even more.
> >
> > ...or just rename dax_zero_page_range() to dax_reset_page_range()?
> > Where reset == "zero + clear-poison"?
>
> I'd say that naming is more confusing than overloading zero.

Ok, I see Darrick has a better suggestion for the shed color.

>
> > > I'm really worried about both patartitions on DAX and DM passing through
> > > DAX because they deeply bind DAX to the block layer, which is just a bad
> > > idea.  I think we also need to sort that whole story out before removing
> > > the EXPERIMENTAL tags.
> >
> > I do think it was a mistake to allow for DAX on partitions of a pmemX
> > block-device.
> >
> > DAX-reflink support may be the opportunity to start deprecating that
> > support. Only enable DAX-reflink for direct mounting on /dev/pmemX
> > without partitions (later add dax-device direct mounting),
>
> I think we need to fully or almost fully sort this out.
>
> Here is my bold suggestions:
>
>  1) drop no drop the EXPERMINTAL on the current block layer overload
>     at all

s/drop no drop/do not drop/?

>  2) add direct mounting of the nvdimm namespaces ASAP.  Because all
>     the filesystem currently also need the /dev/pmem0 device add a way
>     to open the block device by the dax_device instead of our current
>     way of doing the reverse

Oh, interesting. I can get on board with that. There's currently no
/dev entry for namespaces. It's either /dev/pmemX, or /dev/daxX.Y as a
child of /sys/bus/nd/devices/namespaceX.Y. However, I see nothing
glaringly wrong with having /dev/daxX.Y always published regardless of
whether /dev/pmemX is also present.

>  3) deprecate DAX support through block layer mounts with a say 2 year
>     deprecation period
>  4) add DAX remapping devices as needed
>
> I'll volunteer to write the initial code for 2).  And I think we should
> not allow DAX+reflink on the block device shim at all.

Yeah, I think this can fly.