linux-ext4 - Re: [RFC PATCH v2 05/25] ext4: make ext4_map_blocks() distinguish delalloc only extent

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Date: Wed, 3 Jan 2024 21:20:51 +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, tytso@....edu,
 adilger.kernel@...ger.ca, ritesh.list@...il.com, hch@...radead.org,
 djwong@...nel.org, willy@...radead.org, yi.zhang@...wei.com,
 chengzhihao1@...wei.com, yukuai3@...wei.com, wangkefeng.wang@...wei.com
Subject: Re: [RFC PATCH v2 05/25] ext4: make ext4_map_blocks() distinguish
 delalloc only extent

On 2024/1/3 19:31, Jan Kara wrote:
> On Tue 02-01-24 20:38:58, Zhang Yi wrote:
>> From: Zhang Yi <yi.zhang@...wei.com>
>>
>> Add a new map flag EXT4_MAP_DELAYED to indicate the mapping range is a
>> delayed allocated only (not unwritten) one, and making
>> ext4_map_blocks() can distinguish it, no longer mixing it with holes.
>>
>> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
> 
> One small comment below.
> 
>> ---
>>  fs/ext4/ext4.h    | 4 +++-
>>  fs/ext4/extents.c | 5 +++--
>>  fs/ext4/inode.c   | 2 ++
>>  3 files changed, 8 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
>> index a5d784872303..55195909d32f 100644
>> --- a/fs/ext4/ext4.h
>> +++ b/fs/ext4/ext4.h
>> @@ -252,8 +252,10 @@ struct ext4_allocation_request {
>>  #define EXT4_MAP_MAPPED		BIT(BH_Mapped)
>>  #define EXT4_MAP_UNWRITTEN	BIT(BH_Unwritten)
>>  #define EXT4_MAP_BOUNDARY	BIT(BH_Boundary)
>> +#define EXT4_MAP_DELAYED	BIT(BH_Delay)
>>  #define EXT4_MAP_FLAGS		(EXT4_MAP_NEW | EXT4_MAP_MAPPED |\
>> -				 EXT4_MAP_UNWRITTEN | EXT4_MAP_BOUNDARY)
>> +				 EXT4_MAP_UNWRITTEN | EXT4_MAP_BOUNDARY |\
>> +				 EXT4_MAP_DELAYED)
>>  
>>  struct ext4_map_blocks {
>>  	ext4_fsblk_t m_pblk;
>> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
>> index 0892d0568013..fc69f13cf510 100644
>> --- a/fs/ext4/extents.c
>> +++ b/fs/ext4/extents.c
>> @@ -4073,9 +4073,10 @@ static void ext4_ext_determine_hole(struct inode *inode,
>>  	} else if (in_range(map->m_lblk, es.es_lblk, es.es_len)) {
>>  		/*
>>  		 * Straddle the beginning of the queried range, it's no
>> -		 * longer a hole, adjust the length to the delayed extent's
>> -		 * after map->m_lblk.
>> +		 * longer a hole, mark it is a delalloc and adjust the
>> +		 * length to the delayed extent's after map->m_lblk.
>>  		 */
>> +		map->m_flags |= EXT4_MAP_DELAYED;
> 
> I wouldn't set delalloc bit here. If there's delalloc extent containing
> lblk now, it means the caller of ext4_map_blocks() is not holding i_rwsem
> (otherwise we would have found already in ext4_map_blocks()) and thus
> delalloc info is unreliable anyway. So I wouldn't bother. But it's worth a
> comment here like:
> 
> 		/*
> 		 * There's delalloc extent containing lblk. It must have
> 		 * been added after ext4_map_blocks() checked the extent
> 		 * status tree so we are not holding i_rwsem and delalloc
> 		 * info is only stabilized by i_data_sem we are going to
> 		 * release soon. Don't modify the extent status tree and
> 		 * report extent as a hole.
> 		 */
> 

Yeah, the delalloc info is still unreliable. Thanks for the advice, I
will revise it in my next iteration along with your advice in patch 03.

Thanks,
Yi.

> 
>>  		len = es.es_lblk + es.es_len - map->m_lblk;
>>  		goto out;
>>  	} else {
>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
>> index 1b5e6409f958..c141bf6d8db2 100644
>> --- a/fs/ext4/inode.c
>> +++ b/fs/ext4/inode.c
>> @@ -515,6 +515,8 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
>>  			map->m_len = retval;
>>  		} else if (ext4_es_is_delayed(&es) || ext4_es_is_hole(&es)) {
>>  			map->m_pblk = 0;
>> +			map->m_flags |= ext4_es_is_delayed(&es) ?
>> +					EXT4_MAP_DELAYED : 0;
>>  			retval = es.es_len - (map->m_lblk - es.es_lblk);
>>  			if (retval > map->m_len)
>>  				retval = map->m_len;
>> -- 
>> 2.39.2
>>