Re: [PATCH v2 2/3] ext4:Add two functions splitting an extent.

[Yongqiang Yang <xiaoqiangnk@gmail.com>]

On Fri, May 13, 2011 at 10:25 AM, Yongqiang Yang <xiaoqiangnk@gmail.com> wrote:
> On Fri, May 13, 2011 at 5:31 AM, Mingming Cao <cmm@us.ibm.com> wrote:
>> On Mon, 2011-05-02 at 19:05 -0700, Yongqiang Yang wrote:
>>> v0 -> v1:
>>>    -- coding style
>>>    -- try to merge extents in zeroout case too.
>>>
>>> 1] Add a function named ext4_split_extent_at() which splits an extent
>>>    into two extents at given logical block.
>>>
>>> 2] Add a function called ext4_split_extent() which splits an extent
>>>    into three extents.
>>>
>>> Signed-off-by: Yongqiang Yang <xiaoqiangnk@gmail.com>
>>> Tested-by: Allison Henderson <achender@linux.vnet.ibm.com>
>>> ---
>>>  fs/ext4/extents.c |  187 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>>>  1 files changed, 187 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
>>> index 11f30d2..db1d67c 100644
>>> --- a/fs/ext4/extents.c
>>> +++ b/fs/ext4/extents.c
>>> @@ -2554,6 +2554,193 @@ static int ext4_ext_zeroout(struct inode *inode, struct ext4_extent *ex)
>>>       return ret;
>>>  }
>>>
>>> +/*
>>> + * used by extent splitting.
>>> + */
>>> +#define EXT4_EXT_MAY_ZEROOUT 0x1  /* safe to zeroout if split fails \
>>> +                                     due to ENOSPC */
>>> +#define EXT4_EXT_MARK_UNINIT1        0x2  /* mark first half uninitialized */
>>> +#define EXT4_EXT_MARK_UNINIT2        0x4  /* mark second half uninitialized */
>>> +
>>> +/*
>>> + * ext4_split_extent_at() splits an extent at given block.
>>> + *
>>> + * @handle: the journal handle
>>> + * @inode: the file inode
>>> + * @path: the path to the extent
>>> + * @split: the logical block where the extent is splitted.
>>> + * @split_flags: indicates if the extent could be zeroout if split fails, and
>>> + *            the states(init or uninit) of new extents.
>>> + * @flags: flags used to insert new extent to extent tree.
>>> + *
>>> + *
>>> + * Splits extent [a, b] into two extents [a, @split) and [@split, b], states
>>> + * of which are deterimined by split_flag.
>>> + *
>>> + * There are two cases:
>>> + *  a> the extent are splitted into two extent.
>>> + *  b> split is not needed, and just mark the extent.
>>> + *
>>> + * return 0 on success.
>>> + */
>>> +static int ext4_split_extent_at(handle_t *handle,
>>> +                          struct inode *inode,
>>> +                          struct ext4_ext_path *path,
>>> +                          ext4_lblk_t split,
>>> +                          int split_flag,
>>> +                          int flags)
>>> +{
>>> +     ext4_fsblk_t newblock;
>>> +     ext4_lblk_t ee_block;
>>> +     struct ext4_extent *ex, newex, orig_ex;
>>> +     struct ext4_extent *ex2 = NULL;
>>> +     unsigned int ee_len, depth;
>>> +     int err = 0;
>>> +
>>> +     ext_debug("ext4_split_extents_at: inode %lu, logical"
>>> +             "block %llu\n", inode->i_ino, (unsigned long long)split);
>>> +
>>> +     ext4_ext_show_leaf(inode, path);
>>> +
>>> +     depth = ext_depth(inode);
>>> +     ex = path[depth].p_ext;
>>> +     ee_block = le32_to_cpu(ex->ee_block);
>>> +     ee_len = ext4_ext_get_actual_len(ex);
>>> +     newblock = split - ee_block + ext4_ext_pblock(ex);
>>> +
>>> +     BUG_ON(split < ee_block || split >= (ee_block + ee_len));
>>> +
>>> +     err = ext4_ext_get_access(handle, inode, path + depth);
>>> +     if (err)
>>> +             goto out;
>>> +
>>> +     if (split == ee_block) {
>>> +             /*
>>> +              * case b: block @split is the block that the extent begins with
>>> +              * then we just change the state of the extent, and splitting
>>> +              * is not needed.
>>> +              */
>>> +             if (split_flag & EXT4_EXT_MARK_UNINIT2)
>>> +                     ext4_ext_mark_uninitialized(ex);
>>> +             else
>>> +                     ext4_ext_mark_initialized(ex);
>>> +
>>> +             if (!(flags & EXT4_GET_BLOCKS_PRE_IO))
>>> +                     ext4_ext_try_to_merge(inode, path, ex);
>>> +
>>> +             err = ext4_ext_dirty(handle, inode, path + depth);
>>> +             goto out;
>>> +     }
>>> +
>>> +     /* case a */
>>> +     memcpy(&orig_ex, ex, sizeof(orig_ex));
>>> +     ex->ee_len = cpu_to_le16(split - ee_block);
>>> +     if (split_flag & EXT4_EXT_MARK_UNINIT1)
>>> +             ext4_ext_mark_uninitialized(ex);
>>> +
>>> +     /*
>>> +      * path may lead to new leaf, not to original leaf any more
>>> +      * after ext4_ext_insert_extent() returns,
>>> +      */
>>> +     err = ext4_ext_dirty(handle, inode, path + depth);
>>> +     if (err)
>>> +             goto fix_extent_len;
>>> +
>>> +     ex2 = &newex;
>>> +     ex2->ee_block = cpu_to_le32(split);
>>> +     ex2->ee_len   = cpu_to_le16(ee_len - (split - ee_block));
>>> +     ext4_ext_store_pblock(ex2, newblock);
>>> +     if (split_flag & EXT4_EXT_MARK_UNINIT2)
>>> +             ext4_ext_mark_uninitialized(ex2);
>>> +
>>> +     err = ext4_ext_insert_extent(handle, inode, path, &newex, flags);
>>> +     if (err == -ENOSPC && (EXT4_EXT_MAY_ZEROOUT & split_flag)) {
>>> +             err = ext4_ext_zeroout(inode, &orig_ex);
>>> +             if (err)
>>> +                     goto fix_extent_len;
>>> +             /* update the extent length and mark as initialized */
>>> +             ex->ee_len = cpu_to_le32(ee_len);
>>> +             ext4_ext_try_to_merge(inode, path, ex);
>>> +             err = ext4_ext_dirty(handle, inode, path + depth);
>>> +             goto out;
>>> +     } else if (err)
>>> +             goto fix_extent_len;
>>> +
>>> +out:
>>> +     ext4_ext_show_leaf(inode, path);
>>> +     return err;
>>> +
>>> +fix_extent_len:
>>> +     ex->ee_len = orig_ex.ee_len;
>>> +     ext4_ext_dirty(handle, inode, path + depth);
>>> +     return err;
>>> +}
>>> +
>>> +/*
>>> + * ext4_split_extents() splits an extent and mark extent which is covered
>>> + * by @map as split_flags indicates
>>> + *
>>> + * It may result in splitting the extent into multiple extents (upto three)
>>> + * There are three possibilities:
>>> + *   a> There is no split required
>>> + *   b> Splits in two extents: Split is happening at either end of the extent
>>> + *   c> Splits in three extents: Somone is splitting in middle of the extent
>>> + *
>>> + */
>>> +static int ext4_split_extent(handle_t *handle,
>>> +                           struct inode *inode,
>>> +                           struct ext4_ext_path *path,
>>> +                           struct ext4_map_blocks *map,
>>> +                           int split_flag,
>>> +                           int flags)
>>> +{
>>> +     ext4_lblk_t ee_block;
>>> +     struct ext4_extent *ex;
>>> +     unsigned int ee_len, depth;
>>> +     int err = 0;
>>> +     int uninitialized;
>>> +     int split_flag1, flags1;
>>> +
>>> +     depth = ext_depth(inode);
>>> +     ex = path[depth].p_ext;
>>> +     ee_block = le32_to_cpu(ex->ee_block);
>>> +     ee_len = ext4_ext_get_actual_len(ex);
>>> +     uninitialized = ext4_ext_is_uninitialized(ex);
>>> +
>>> +     if (map->m_lblk + map->m_len < ee_block + ee_len) {
>>> +             split_flag1 = split_flag & EXT4_EXT_MAY_ZEROOUT ?
>>> +                           EXT4_EXT_MAY_ZEROOUT : 0;
>>> +             flags1 = flags | EXT4_GET_BLOCKS_PRE_IO;
>>> +             if (uninitialized)
>>> +                     split_flag1 |= EXT4_EXT_MARK_UNINIT1 |
>>> +                                    EXT4_EXT_MARK_UNINIT2;
>>> +             err = ext4_split_extent_at(handle, inode, path,
>>> +                             map->m_lblk + map->m_len, split_flag1, flags1);
>>> +     }
>>> +
> Thank you for looking into the patch.
>
> First, I think I need to explain split_flag added in the patch.  There
> are three flags in split_flag1.
>
> 1.EXT4_EXT_MAY_ZEROOUT means whole extent can be zeroouted instead of
> splitting when splitting fails due to ENOSPACE;
>
> 2.EXT4_EXT_MARK_UNINIT1 indicates that the 1st part extent should be
> marked uninitialized, otherwise initialized.
>
> 3.EXT4_EXT_MARK_UNINIT2 is similar to EXT4_EXT_MARK_UNINIT1, only
> difference is that it has effect on the 2nd extent.
>>
>> Hmm, I could not see the zeroout extent gets marked as initialized here.
>> Nothing wrong to expose the wrong data, but certainly we are not take
>> advantage of zero out,  Perhaps I missed something?
>
> Here the extent [ee_block, ee_block + ee_len) is split into two
> extents [ee_block, map->m_lblk + map->m_len) and [map->m_lblk +
> map->m_len, ee_block + ee_len), which are marked same as ex.
>>
>> It would be nice to add some comments to describe the difference of
>> split_flag1, flags1, flags:-) Thanks.
>>
>> Also, I think we miss error handling here. What if the first split
>> failed and return error here? we still proceed to to do next split? I
>> think we should go to the err exit, isnt?
> Yes, error handling is missed.
>>
>>
>>> +     ext4_ext_drop_refs(path);
>>> +     path = ext4_ext_find_extent(inode, map->m_lblk, path);
>>> +     if (IS_ERR(path))
>>> +             return PTR_ERR(path);
>>> +
>>> +     if (map->m_lblk >= ee_block) {
When map->m_lblk == ee_block, [ee_block, ee_block + ee_len) is treated
as 2nd extent.
>>> +             split_flag1 = split_flag & EXT4_EXT_MAY_ZEROOUT ?
>>> +                           EXT4_EXT_MAY_ZEROOUT : 0;
>>> +             if (uninitialized)
>>> +                     split_flag1 |= EXT4_EXT_MARK_UNINIT1;
>>> +             if (split_flag & EXT4_EXT_MARK_UNINIT2)
>>> +                     split_flag1 |= EXT4_EXT_MARK_UNINIT2;
> Up to now, zeroout part will be marked initialized.
>>> +             err = ext4_split_extent_at(handle, inode, path,
>>> +                             map->m_lblk, split_flag1, flags);
>>> +             if (err)
>>> +                     goto out;
>>> +     }
>
>>> +
>>> +     ext4_ext_show_leaf(inode, path);
>>> +out:
>>> +     return err ? err : map->m_len;
>>> +}
>>> +
>>>  #define EXT4_EXT_ZERO_LEN 7
>>>  /*
>>>   * This function is called by ext4_ext_map_blocks() if someone tries to write
>>
>>
>>
>
>
>
> --
> Best Wishes
> Yongqiang Yang
>



-- 
Best Wishes
Yongqiang Yang
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