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

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Message-ID: <a904fae9-68ce-6035-8aa2-5d43a3882d79@oracle.com>
Date:   Sat, 26 Mar 2022 06:31:36 +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.
> 
>>>> 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.
> 

Getting dax_direct_access to return pfn seems straight forward,
what do you think of below change?

--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -195,10 +195,10 @@ int dax_zero_page_range(struct dax_device 
*dax_dev, pgoff_t pgoff,
  }
  EXPORT_SYMBOL_GPL(dax_zero_page_range);

-size_t dax_recovery_write(struct dax_device *dax_dev, pgoff_t pgoff,
+size_t dax_recovery_write(struct dax_device *dax_dev, pgoff_t pgoff, 
pfn_t pfn,
                 void *addr, size_t bytes, struct iov_iter *iter)
  {
-       struct dev_pagemap *pgmap = dax_dev->pgmap;
+       struct dev_pagemap *pgmap = get_dev_pagemap(pfn_t_to_pfn(pfn), 
NULL);

         if (!pgmap || !pgmap->ops->recovery_write)
                 return -EIO;


--- a/fs/dax.c
+++ b/fs/dax.c
@@ -1243,6 +1243,7 @@ static loff_t dax_iomap_iter(const struct 
iomap_iter *iomi,
                 int flags, recov;
                 void *kaddr;
                 long nrpg;
+               pfn_t pfn;

                 if (fatal_signal_pending(current)) {
                         ret = -EINTR;
@@ -1257,7 +1258,7 @@ static loff_t dax_iomap_iter(const struct 
iomap_iter *iomi,
                 if ((map_len == -EIO) && (iov_iter_rw(iter) == WRITE)) {
                         flags |= DAX_RECOVERY;
                         map_len = dax_direct_access(dax_dev, pgoff, nrpg,
-                                               flags, &kaddr, NULL);
+                                               flags, &kaddr, &pfn);
                         if (map_len > 0)
                                 recov++;
                 }
@@ -1273,7 +1274,7 @@ static loff_t dax_iomap_iter(const struct 
iomap_iter *iomi,
                         map_len = end - pos;

                 if (recov)
-                       xfer = dax_recovery_write(dax_dev, pgoff, kaddr,
+                       xfer = dax_recovery_write(dax_dev, pgoff, pfn, 
kaddr,
                                         map_len, iter);
                 else if (iov_iter_rw(iter) == WRITE)
                         xfer = dax_copy_from_iter(dax_dev, pgoff, kaddr,


thanks!
-jane