linux-kernel - Re: [PATCH v6 4/6] dax: add DAX_RECOVERY flag and .recovery_write dev_pgmap

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Date:   Tue, 22 Mar 2022 02:01:40 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jane Chu <jane.chu@...cle.com>
Cc:     david@...morbit.com, djwong@...nel.org, dan.j.williams@...el.com,
        hch@...radead.org, vishal.l.verma@...el.com, dave.jiang@...el.com,
        agk@...hat.com, snitzer@...hat.com, dm-devel@...hat.com,
        ira.weiny@...el.com, willy@...radead.org, vgoyal@...hat.com,
        linux-fsdevel@...r.kernel.org, nvdimm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org
Subject: Re: [PATCH v6 4/6] dax: add DAX_RECOVERY flag and .recovery_write
 dev_pgmap_ops

On Sat, Mar 19, 2022 at 12:28:31AM -0600, Jane Chu wrote:
> Introduce DAX_RECOVERY flag to dax_direct_access(). The flag is
> not set by default in dax_direct_access() such that the helper
> does not translate a pmem range to kernel virtual address if the
> range contains uncorrectable errors.  When the flag is set,
> the helper ignores the UEs and return kernel virtual adderss so
> that the caller may get on with data recovery via write.

This DAX_RECOVERY doesn't actually seem to be used anywhere here or
in the subsequent patches.  Did I miss something?

> Also introduce a new dev_pagemap_ops .recovery_write function.
> The function is applicable to FSDAX device only. The device
> page backend driver provides .recovery_write function if the
> device has underlying mechanism to clear the uncorrectable
> errors on the fly.

Why is this not in struct dax_operations?

>  
> +size_t dax_recovery_write(struct dax_device *dax_dev, pgoff_t pgoff,
> +		void *addr, size_t bytes, struct iov_iter *iter)
> +{
> +	struct dev_pagemap *pgmap = dax_dev->pgmap;
> +
> +	if (!pgmap || !pgmap->ops->recovery_write)
> +		return -EIO;
> +	return pgmap->ops->recovery_write(pgmap, pgoff, addr, bytes,
> +				(void *)iter);

No need to cast a type pointer to a void pointer.  But more importantly
losing the type information here and passing it as void seems very
wrong.

> +static size_t pmem_recovery_write(struct dev_pagemap *pgmap, pgoff_t pgoff,
> +		void *addr, size_t bytes, void *iter)
> +{
> +	struct pmem_device *pmem = pgmap->owner;
> +
> +	dev_warn(pmem->bb.dev, "%s: not yet implemented\n", __func__);
> +
> +	/* XXX more later */
> +	return 0;
> +}

This shuld not be added here - the core code can cope with a NULL
method just fine.

> +		recov = 0;
> +		flags = 0;
> +		nrpg = PHYS_PFN(size);

Please spell out the words.  The recovery flag can also be
a bool to make the code more readable.

> +		map_len = dax_direct_access(dax_dev, pgoff, nrpg, flags,
> +					&kaddr, NULL);
> +		if ((map_len == -EIO) && (iov_iter_rw(iter) == WRITE)) {

No need for the inner braces.

> +			flags |= DAX_RECOVERY;
> +			map_len = dax_direct_access(dax_dev, pgoff, nrpg,
> +						flags, &kaddr, NULL);

And noneed for the flags variable at all really.

>  			xfer = dax_copy_from_iter(dax_dev, pgoff, kaddr,
>  					map_len, iter);
>  		else
> @@ -1271,6 +1286,11 @@ static loff_t dax_iomap_iter(const struct iomap_iter *iomi,
>  		length -= xfer;
>  		done += xfer;
>  
> +		if (recov && (xfer == (ssize_t) -EIO)) {
> +			pr_warn("dax_recovery_write failed\n");
> +			ret = -EIO;
> +			break;

And no, we can't just use an unsigned variable to communicate a
negative error code.