linux-kernel - Re: [PATCH v2 03/13] ext4: don't zero the entire extent if EXT4_EXT_DATA_PARTIAL

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Message-ID: <c3e847cd-064c-4bff-a464-be8d1b8fc778@huaweicloud.com>
Date: Thu, 27 Nov 2025 14:13:05 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
Cc: linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, tytso@....edu, adilger.kernel@...ger.ca,
 jack@...e.cz, yi.zhang@...wei.com, yizhang089@...il.com,
 libaokun1@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH v2 03/13] ext4: don't zero the entire extent if
 EXT4_EXT_DATA_PARTIAL_VALID1

On 11/26/2025 7:29 PM, Ojaswin Mujoo wrote:
> On Fri, Nov 21, 2025 at 02:08:01PM +0800, Zhang Yi wrote:
>> From: Zhang Yi <yi.zhang@...wei.com>
>>
>> When allocating initialized blocks from a large unwritten extent, or
>> when splitting an unwritten extent during end I/O and converting it to
>> initialized, there is currently a potential issue of stale data if the
>> extent needs to be split in the middle.
>>
>>        0  A      B  N
>>        [UUUUUUUUUUUU]    U: unwritten extent
>>        [--DDDDDDDD--]    D: valid data
>>           |<-  ->| ----> this range needs to be initialized
>>
>> ext4_split_extent() first try to split this extent at B with
>> EXT4_EXT_DATA_ENTIRE_VALID1 and EXT4_EXT_MAY_ZEROOUT flag set, but
>> ext4_split_extent_at() failed to split this extent due to temporary lack
>> of space. It zeroout B to N and mark the entire extent from 0 to N
>> as written.
>>
>>        0  A      B  N
>>        [WWWWWWWWWWWW]    W: written extent
>>        [SSDDDDDDDDZZ]    Z: zeroed, S: stale data
>>
>> ext4_split_extent() then try to split this extent at A with
>> EXT4_EXT_DATA_VALID2 flag set. This time, it split successfully and left
>> a stale written extent from 0 to A.
>>
>>        0  A      B   N
>>        [WW|WWWWWWWWWW]
>>        [SS|DDDDDDDDZZ]
>>
>> Fix this by pass EXT4_EXT_DATA_PARTIAL_VALID1 to ext4_split_extent_at()
>> when splitting at B, don't convert the entire extent to written and left
>> it as unwritten after zeroing out B to N. The remaining work is just
>> like the standard two-part split. ext4_split_extent() will pass the
>> EXT4_EXT_DATA_VALID2 flag when it calls ext4_split_extent_at() for the
>> second time, allowing it to properly handle the split. If the split is
>> successful, it will keep extent from 0 to A as unwritten.
>>
>> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
> 
> Hi Yi,
> 
> This patch looks good to me. I'm just wondering since this is a stale
> data exposure that might need a backport, should we add a Fixes: tag 
> and also keep these fixes before the refactor in 1/13 so backport is
> easier.

Sure, I can move patch 01 after all the fix patches.

Thanks,
Yi.

> 
> Other than that, feel free to add:
> Reviewed-by: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
> 
> Regards,
> ojaswin
> 
>> ---
>>  fs/ext4/extents.c | 11 ++++++++++-
>>  1 file changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
>> index f7aa497e5d6c..cafe66cb562f 100644
>> --- a/fs/ext4/extents.c
>> +++ b/fs/ext4/extents.c
>> @@ -3294,6 +3294,13 @@ static struct ext4_ext_path *ext4_split_extent_at(handle_t *handle,
>>  		err = ext4_ext_zeroout(inode, &zero_ex);
>>  		if (err)
>>  			goto fix_extent_len;
>> +		/*
>> +		 * The first half contains partially valid data, the splitting
>> +		 * of this extent has not been completed, fix extent length
>> +		 * and ext4_split_extent() split will the first half again.
>> +		 */
>> +		if (split_flag & EXT4_EXT_DATA_PARTIAL_VALID1)
>> +			goto fix_extent_len;
>>  
>>  		/* update the extent length and mark as initialized */
>>  		ex->ee_len = cpu_to_le16(ee_len);
>> @@ -3364,7 +3371,9 @@ static struct ext4_ext_path *ext4_split_extent(handle_t *handle,
>>  			split_flag1 |= EXT4_EXT_MARK_UNWRIT1 |
>>  				       EXT4_EXT_MARK_UNWRIT2;
>>  		if (split_flag & EXT4_EXT_DATA_VALID2)
>> -			split_flag1 |= EXT4_EXT_DATA_ENTIRE_VALID1;
>> +			split_flag1 |= map->m_lblk > ee_block ?
>> +				       EXT4_EXT_DATA_PARTIAL_VALID1 :
>> +				       EXT4_EXT_DATA_ENTIRE_VALID1;
>>  		path = ext4_split_extent_at(handle, inode, path,
>>  				map->m_lblk + map->m_len, split_flag1, flags1);
>>  		if (IS_ERR(path))
>> -- 
>> 2.46.1
>>