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


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