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

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On Fri, May 13, 2011 at 5:31 AM, Mingming Cao <cmm@xxxxxxxxxx> 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@xxxxxxxxx>
>> Tested-by: Allison Henderson <achender@xxxxxxxxxxxxxxxxxx>
>> ---
>>  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) {
>> +             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
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