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

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Date:   Fri, 17 Sep 2021 08:27:44 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        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 Fri, Sep 17, 2021 at 01:53:33PM +0100, Christoph Hellwig 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.

How about dax_zeroinit_range() ?

To go with its fallocate flag (yeah I've been too busy sorting out -rc1
regressions to repost this) FALLOC_FL_ZEROINIT_RANGE that will reset the
hardware (whatever that means) and set the contents to the known value
zero.

Userspace usage model:

void handle_media_error(int fd, loff_t pos, size_t len)
{
	/* yell about this for posterior's sake */

	ret = fallocate(fd, FALLOC_FL_ZEROINIT_RANGE, pos, len);

	/* yay our disk drive / pmem / stone table engraver is online */
}

> > > 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

I don't understand this.

>  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

What devices are needed?  linear for lvm, and maybe error so we can
actually test all this stuff?

> 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.

/me has other questions about daxreflink, but I'll ask them on shiyang's
thread.

--D