Re: [PATCH v7 7/8] fsdax: Introduce dax_iomap_ops for end of reflink

[Shiyang Ruan <ruansy.fnst@fujitsu.com>]

On 2021/8/27 13:04, Dan Williams wrote:
> On Thu, Aug 26, 2021 at 8:30 PM Shiyang Ruan <ruansy.fnst@fujitsu.com> wrote:
>>
>>
>>
>> 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.
> 
> Correct, but you can use container_of() to get back to the iter and
> consider the raw untranslated value of iter->processed. As Christoph
> mentioned this needs a comment explaining the layering violation, but
> that's a cleaner change than the dax_iomap_ops approach.
> 

Understood.  Thanks.

--
Ruan.