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

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Message-ID: <YUSPzVG0ulHdLWn7@infradead.org>
Date:   Fri, 17 Sep 2021 13:53:33 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "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>
Subject: Re: [PATCH 0/3] dax: clear poison on the fly along pwrite

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.

> > 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
 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
 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.