linux-kernel - Re: [PATCH v2 2/2] dax,pmem: Implement pmem based dax data recovery

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Date:   Tue, 9 Nov 2021 19:14:52 +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 v2 2/2] dax,pmem: Implement pmem based dax data recovery

On 11/8/2021 11:27 PM, Christoph Hellwig wrote:
> On Fri, Nov 05, 2021 at 07:16:38PM -0600, Jane Chu wrote:
>>   static size_t pmem_copy_from_iter(struct dax_device *dax_dev, pgoff_t pgoff,
>>   		void *addr, size_t bytes, struct iov_iter *i, int mode)
>>   {
>> +	phys_addr_t pmem_off;
>> +	size_t len, lead_off;
>> +	struct pmem_device *pmem = dax_get_private(dax_dev);
>> +	struct device *dev = pmem->bb.dev;
>> +
>> +	if (unlikely(mode == DAX_OP_RECOVERY)) {
>> +		lead_off = (unsigned long)addr & ~PAGE_MASK;
>> +		len = PFN_PHYS(PFN_UP(lead_off + bytes));
>> +		if (is_bad_pmem(&pmem->bb, PFN_PHYS(pgoff) / 512, len)) {
>> +			if (lead_off || !(PAGE_ALIGNED(bytes))) {
>> +				dev_warn(dev, "Found poison, but addr(%p) and/or bytes(%#lx) not page aligned\n",
>> +					addr, bytes);
>> +				return (size_t) -EIO;
>> +			}
>> +			pmem_off = PFN_PHYS(pgoff) + pmem->data_offset;
>> +			if (pmem_clear_poison(pmem, pmem_off, bytes) !=
>> +						BLK_STS_OK)
>> +				return (size_t) -EIO;
>> +		}
>> +	}
> 
> This is in the wrong spot.  As seen in my WIP series individual drivers
> really should not hook into copying to and from the iter, because it
> really is just one way to write to a nvdimm.  How would dm-writecache
> clear the errors with this scheme?

How does dm-writecache detect and clear errors today?

> 
> So IMHO going back to the separate recovery method as in your previous
> patch really is the way to go.  If/when the 64-bit store happens we
> need to figure out a good way to clear the bad block list for that.
> 
Yeah, the separate .dax_clear_poison API may not be arbitrarily called
though. It must be followed by a 'write' operation, and so, unless we
bind the two operations together, how do we enforce that to avoid
silent data corruption?

thanks!
-jane