linux-kernel - Re: [PATCH v5 2/7] dax: introduce dax device flag DAXDEV

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Date:   Wed, 2 Feb 2022 21:27:42 +0000
From:   Jane Chu <jane.chu@...cle.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     "david@...morbit.com" <david@...morbit.com>,
        "djwong@...nel.org" <djwong@...nel.org>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "vishal.l.verma@...el.com" <vishal.l.verma@...el.com>,
        "dave.jiang@...el.com" <dave.jiang@...el.com>,
        "agk@...hat.com" <agk@...hat.com>,
        "snitzer@...hat.com" <snitzer@...hat.com>,
        "dm-devel@...hat.com" <dm-devel@...hat.com>,
        "ira.weiny@...el.com" <ira.weiny@...el.com>,
        "willy@...radead.org" <willy@...radead.org>,
        "vgoyal@...hat.com" <vgoyal@...hat.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "nvdimm@...ts.linux.dev" <nvdimm@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>
Subject: Re: [PATCH v5 2/7] dax: introduce dax device flag DAXDEV_RECOVERY

On 2/2/2022 5:23 AM, Christoph Hellwig wrote:
> On Fri, Jan 28, 2022 at 02:31:45PM -0700, Jane Chu wrote:
>> +int dax_prep_recovery(struct dax_device *dax_dev, void **kaddr)
>> +{
>> +	if (dax_recovery_capable(dax_dev)) {
>> +		set_bit(DAXDEV_RECOVERY, (unsigned long *)kaddr);
>> +		return 0;
>> +	}
>> +	return -EINVAL;
> 
> Setting a random bit on a passed in memory address looks a little
> dangerous to me.

Yeah, I see.  Would you suggest a way to pass the indication from
dax_iomap_iter to dax_direct_access that the caller intends the
callee to ignore poison in the range because the caller intends
to do recovery_write? We tried adding a flag to dax_direct_access, and 
that wasn't liked if I recall.

> 
> Also I'd return early for the EINVAL case to make the flow a little
> more clear.

Agreed, will do.

thanks!
-jane