linux-kernel - Re: [PATCH -next 3/7] ext4: avoid starting handle when dio writing an unwritten extent

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Message-ID: <5f6f9588-52a0-4ab8-a1ad-3d466488b985@huaweicloud.com>
Date: Sat, 20 Dec 2025 15:16:41 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: Jan Kara <jack@...e.cz>
Cc: linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, tytso@....edu, adilger.kernel@...ger.ca,
 ojaswin@...ux.ibm.com, ritesh.list@...il.com, yi.zhang@...wei.com,
 yizhang089@...il.com, libaokun1@...wei.com, yangerkun@...wei.com,
 yukuai@...as.com
Subject: Re: [PATCH -next 3/7] ext4: avoid starting handle when dio writing an
 unwritten extent

On 12/19/2025 11:25 PM, Jan Kara wrote:
> On Sat 13-12-25 10:20:04, Zhang Yi wrote:
>> From: Zhang Yi <yi.zhang@...wei.com>
>>
>> Since we have deferred the split of the unwritten extent until after I/O
>> completion, it is not necessary to initiate the journal handle when
>> submitting the I/O.
>>
>> This can improve the write performance of concurrent DIO for multiple
>> files. The fio tests below show a ~25% performance improvement when
>> wirting to unwritten files on my VM with a mem disk.
>>
>>   [unwritten]
>>   direct=1
>>   ioengine=psync
>>   numjobs=16
>>   rw=write     # write/randwrite
>>   bs=4K
>>   iodepth=1
>>   directory=/mnt
>>   size=5G
>>   runtime=30s
>>   overwrite=0
>>   norandommap=1
>>   fallocate=native
>>   ramp_time=5s
>>   group_reporting=1
>>
>>  [w/o]
>>   w:  IOPS=62.5k, BW=244MiB/s
>>   rw: IOPS=56.7k, BW=221MiB/s
>>
>>  [w]
>>   w:  IOPS=79.6k, BW=311MiB/s
>>   rw: IOPS=70.2k, BW=274MiB/s
>>
>> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
>> ---
>>  fs/ext4/file.c  | 4 +---
>>  fs/ext4/inode.c | 4 +++-
>>  2 files changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
>> index 7a8b30932189..9f571acc7782 100644
>> --- a/fs/ext4/file.c
>> +++ b/fs/ext4/file.c
>> @@ -418,9 +418,7 @@ static const struct iomap_dio_ops ext4_dio_write_ops = {
>>   *   updating inode i_disksize and/or orphan handling with exclusive lock.
>>   *
>>   * - shared locking will only be true mostly with overwrites, including
>> - *   initialized blocks and unwritten blocks. For overwrite unwritten blocks
>> - *   we protect splitting extents by i_data_sem in ext4_inode_info, so we can
>> - *   also release exclusive i_rwsem lock.
>> + *   initialized blocks and unwritten blocks.
>>   *
>>   * - Otherwise we will switch to exclusive i_rwsem lock.
>>   */
>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
>> index ffde24ff7347..08a296122fe0 100644
>> --- a/fs/ext4/inode.c
>> +++ b/fs/ext4/inode.c
>> @@ -3819,7 +3819,9 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
>>  			 * For atomic writes the entire requested length should
>>  			 * be mapped.
>>  			 */
>> -			if (map.m_flags & EXT4_MAP_MAPPED) {
>> +			if ((map.m_flags & EXT4_MAP_MAPPED) ||
>> +			    (!(flags & IOMAP_DAX) &&
> 
> Why is here an exception for DAX writes? DAX is fine writing to unwritten
> extents AFAIK. It only needs to pre-zero newly allocated blocks... Or am I
> missing some corner case?
> 
> 								Honza

Hi, Jan!

Thank you for reviewing this series.

Yes, that is precisely why this exception is necessary here. Without this
exception, a DAX write to an unwritten extent would return immediately
without invoking ext4_iomap_alloc() to perform pre-zeroing.

Thanks,
Yi.

> 
>> +			     (map.m_flags & EXT4_MAP_UNWRITTEN))) {
>>  				if ((!(flags & IOMAP_ATOMIC) && ret > 0) ||
>>  				   (flags & IOMAP_ATOMIC && ret >= orig_mlen))
>>  					goto out;
>> -- 
>> 2.46.1
>>