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

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Date:   Wed, 23 Mar 2022 18:43:06 +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 v6 4/6] dax: add DAX_RECOVERY flag and .recovery_write
 dev_pgmap_ops

On 3/22/2022 10:45 PM, Christoph Hellwig wrote:
> On Tue, Mar 22, 2022 at 11:05:09PM +0000, Jane Chu wrote:
>>> This DAX_RECOVERY doesn't actually seem to be used anywhere here or
>>> in the subsequent patches.  Did I miss something?
>>
>> dax_iomap_iter() uses the flag in the same patch,
>> +               if ((map_len == -EIO) && (iov_iter_rw(iter) == WRITE)) {
>> +                       flags |= DAX_RECOVERY;
>> +                       map_len = dax_direct_access(dax_dev, pgoff, nrpg,
>> +                                               flags, &kaddr, NULL);
> 
> Yes, it passes it on to dax_direct_access, and dax_direct_access passes
> it onto ->direct_access.  But nothing in this series actually checks
> for it as far as I can tell.

The flag is checked here, again, I'll spell out the flag rather than 
using it as a boolean.

  __weak long __pmem_direct_access(struct pmem_device *pmem, pgoff_t pgoff,
-		long nr_pages, void **kaddr, pfn_t *pfn)
+		long nr_pages, int flags, void **kaddr, pfn_t *pfn)
  {
  	resource_size_t offset = PFN_PHYS(pgoff) + pmem->data_offset;

-	if (unlikely(is_bad_pmem(&pmem->bb, PFN_PHYS(pgoff) / 512,
+	if (!flags && unlikely(is_bad_pmem(&pmem->bb, PFN_PHYS(pgoff) / 512,
  					PFN_PHYS(nr_pages))))
  		return -EIO;

> 
>>>> 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?
>>
>> Per Dan's comments to the v5 series, adding .recovery_write to
>> dax_operations causes a number of trivial dm targets changes.
>> Dan suggested that adding .recovery_write to pagemap_ops could
>> cut short the logistics of figuring out whether the driver backing
>> up a page is indeed capable of clearing poison. Please see
>> https://lkml.org/lkml/2022/2/4/31
> 
> But at least in this series there is  1:1 association between the
> pgmap and the dax_device so that scheme won't work.   It would
> have to lookup the pgmap based on the return physical address from
> dax_direct_access.  Which sounds more complicated than just adding
> the (annoying) boilerplate code to DM.
> 

Yes, good point!  Let me look into this.

>> include/linux/memremap.h doesn't know struct iov_iter which is defined
>> in include/linux/uio.h,  would you prefer to adding include/linux/uio.h
>> to include/linux/memremap.h ?
> 
> As it is not derefences just adding a
> 
> struct iov_iter;
> 
> line to memremap.h below the includes should be all that is needed.

Sure, will do.

Thanks!
-jane