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Date: Fri, 27 Aug 2021 13:27:55 +0800
From: Shiyang Ruan <ruansy.fnst@...itsu.com>
To: Dan Williams <dan.j.williams@...el.com>
CC: "Darrick J. Wong" <djwong@...nel.org>,
Christoph Hellwig <hch@....de>,
linux-xfs <linux-xfs@...r.kernel.org>,
david <david@...morbit.com>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Linux NVDIMM <nvdimm@...ts.linux.dev>,
Goldwyn Rodrigues <rgoldwyn@...e.de>,
Al Viro <viro@...iv.linux.org.uk>,
Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v7 7/8] fsdax: Introduce dax_iomap_ops for end of reflink
On 2021/8/27 13:04, Dan Williams wrote:
> On Thu, Aug 26, 2021 at 8:30 PM Shiyang Ruan <ruansy.fnst@...itsu.com> wrote:
>>
>>
>>
>> On 2021/8/20 23:18, Dan Williams wrote:
>>> On Thu, Aug 19, 2021 at 11:13 PM ruansy.fnst <ruansy.fnst@...itsu.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@...itsu.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@...itsu.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.
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