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Subject: Re: [PATCH v7 7/8] fsdax: Introduce dax_iomap_ops for end of reflink
To:     Dan Williams <dan.j.williams@intel.com>
CC:     "Darrick J. Wong" <djwong@kernel.org>,
        Christoph Hellwig <hch@lst.de>,
        linux-xfs <linux-xfs@vger.kernel.org>,
        david <david@fromorbit.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux NVDIMM <nvdimm@lists.linux.dev>,
        Goldwyn Rodrigues <rgoldwyn@suse.de>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Matthew Wilcox <willy@infradead.org>
References: <20210816060359.1442450-1-ruansy.fnst@fujitsu.com>
 <20210816060359.1442450-8-ruansy.fnst@fujitsu.com>
 <CAPcyv4jbi=p=SjFYZcHnEAu+KY821pW_k_yA5u6hya4jEfrTUg@mail.gmail.com>
 <c7e68dc8-5a43-f727-c262-58dcf244c711@fujitsu.com>
 <CAPcyv4jM86gy-T5EEZf6M2m44v4MiGqYDhxisX59M5QJii6DVg@mail.gmail.com>
From:   Shiyang Ruan <ruansy.fnst@fujitsu.com>
Message-ID: <32fa5333-b14e-2060-d659-d77f6c75ff16@fujitsu.com>
Date:   Fri, 27 Aug 2021 11:29:54 +0800
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On 2021/8/20 23:18, Dan Williams wrote:
> On Thu, Aug 19, 2021 at 11:13 PM ruansy.fnst <ruansy.fnst@fujitsu.com> wrote:
>>
>>
>>
>> On 2021/8/20 上午11:01, Dan Williams wrote:
>>> On Sun, Aug 15, 2021 at 11:05 PM Shiyang Ruan <ruansy.fnst@fujitsu.com> wrote:
>>>>
>>>> After writing data, reflink requires end operations to remap those new
>>>> allocated extents.  The current ->iomap_end() ignores the error code
>>>> returned from ->actor(), so we introduce this dax_iomap_ops and change
>>>> the dax_iomap_*() interfaces to do this job.
>>>>
>>>> - the dax_iomap_ops contains the original struct iomap_ops and fsdax
>>>>       specific ->actor_end(), which is for the end operations of reflink
>>>> - also introduce dax specific zero_range, truncate_page
>>>> - create new dax_iomap_ops for ext2 and ext4
>>>>
>>>> Signed-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
>>>> ---
>>>>    fs/dax.c               | 68 +++++++++++++++++++++++++++++++++++++-----
>>>>    fs/ext2/ext2.h         |  3 ++
>>>>    fs/ext2/file.c         |  6 ++--
>>>>    fs/ext2/inode.c        | 11 +++++--
>>>>    fs/ext4/ext4.h         |  3 ++
>>>>    fs/ext4/file.c         |  6 ++--
>>>>    fs/ext4/inode.c        | 13 ++++++--
>>>>    fs/iomap/buffered-io.c |  3 +-
>>>>    fs/xfs/xfs_bmap_util.c |  3 +-
>>>>    fs/xfs/xfs_file.c      |  8 ++---
>>>>    fs/xfs/xfs_iomap.c     | 36 +++++++++++++++++++++-
>>>>    fs/xfs/xfs_iomap.h     | 33 ++++++++++++++++++++
>>>>    fs/xfs/xfs_iops.c      |  7 ++---
>>>>    fs/xfs/xfs_reflink.c   |  3 +-
>>>>    include/linux/dax.h    | 21 ++++++++++---
>>>>    include/linux/iomap.h  |  1 +
>>>>    16 files changed, 189 insertions(+), 36 deletions(-)
>>>>
>>>> diff --git a/fs/dax.c b/fs/dax.c
>>>> index 74dd918cff1f..0e0536765a7e 100644
>>>> --- a/fs/dax.c
>>>> +++ b/fs/dax.c
>>>> @@ -1348,11 +1348,30 @@ static loff_t dax_iomap_iter(const struct iomap_iter *iomi,
>>>>           return done ? done : ret;
>>>>    }
>>>>
>>>> +static inline int
>>>> +__dax_iomap_iter(struct iomap_iter *iter, const struct dax_iomap_ops *ops)
>>>> +{
>>>> +       int ret;
>>>> +
>>>> +       /*
>>>> +        * Call dax_iomap_ops->actor_end() before iomap_ops->iomap_end() in
>>>> +        * each iteration.
>>>> +        */
>>>> +       if (iter->iomap.length && ops->actor_end) {
>>>> +               ret = ops->actor_end(iter->inode, iter->pos, iter->len,
>>>> +                                    iter->processed);
>>>> +               if (ret < 0)
>>>> +                       return ret;
>>>> +       }
>>>> +
>>>> +       return iomap_iter(iter, &ops->iomap_ops);
>>>
>>> This reorganization looks needlessly noisy. Why not require the
>>> iomap_end operation to perform the actor_end work. I.e. why can't
>>> xfs_dax_write_iomap_actor_end() just be the passed in iomap_end? I am
>>> not seeing where the ->iomap_end() result is ignored?
>>>
>>
>> The V6 patch[1] was did in this way.
>> [1]https://lore.kernel.org/linux-xfs/20210526005159.GF202144@locust/T/#m79a66a928da2d089e2458c1a97c0516dbfde2f7f
>>
>> But Darrick reminded me that ->iomap_end() will always take zero or
>> positive 'written' because iomap_apply() handles this argument.
>>
>> ```
>>          if (ops->iomap_end) {
>>                  ret = ops->iomap_end(inode, pos, length,
>>                                       written > 0 ? written : 0,
>>                                       flags, &iomap);
>>          }
>> ```
>>
>> So, we cannot get actual return code from CoW in ->actor(), and as a
>> result, we cannot handle the xfs end_cow correctly in ->iomap_end().
>> That's where the result of CoW was ignored.
> 
> Ah, thank you for the explanation.
> 
> However, this still seems like too much code thrash just to get back
> to the original value of iter->processed. I notice you are talking
> about iomap_apply(), but that routine is now gone in Darrick's latest
> iomap-for-next branch. Instead iomap_iter() does this:
> 
>          if (iter->iomap.length && ops->iomap_end) {
>                  ret = ops->iomap_end(iter->inode, iter->pos, iomap_length(iter),
>                                  iter->processed > 0 ? iter->processed : 0,

As you can see, here is the same logic as the old iomap_apply(): the 
negative iter->processed won't be passed into ->iomap_end().

>                                  iter->flags, &iter->iomap);
>                  if (ret < 0 && !iter->processed)
>                          return ret;
>          }
> 
> 
> I notice that the @iomap argument to ->iomap_end() is reliably coming
> from @iter. So you could do the following in your iomap_end()
> callback:
> 
>          struct iomap_iter *iter = container_of(iomap, typeof(*iter), iomap);
>          struct xfs_inode *ip = XFS_I(inode);
>          ssize_t written = iter->processed;

The written will be 0 or positive.  The original error code is ingnored.

>          bool cow = xfs_is_cow_inode(ip);
> 
>          if (cow) {
>                  if (written <= 0)
>                          xfs_reflink_cancel_cow_range(ip, pos, length, true)
>          }
> 

--
Thanks,
Ruan.