linux-ext4 - Re: [PATCH RFC 4/4] btrfs-progs: convert: support ext2 unwritten file data extents

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Message-ID: <7a33ff31-e570-4775-b82f-1c6413656699@oracle.com>
Date: Sat, 4 May 2024 07:27:46 +0800
From: Anand Jain <anand.jain@...cle.com>
To: Qu Wenruo <wqu@...e.com>, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org
Subject: Re: [PATCH RFC 4/4] btrfs-progs: convert: support ext2 unwritten file
 data extents



On 5/4/24 06:23, Qu Wenruo wrote:
> 
> 
> 在 2024/5/3 21:55, Anand Jain 写道:
> [...]
>>>> +int find_prealloc(struct blk_iterate_data *data, int index, bool 
>>>> *prealloc)
>>>
>>> This function is called for every file extent we're going to create.
>>> I'm not a big fan of doing so many lookup.
>>>
>>> My question is, is this the only way to determine the flag of the 
>>> data extent?
>>>
>>> My instinct says there should be a straight forward way to determine 
>>> if a file extent is preallocated or not, just like what we do in our 
>>> file extent items.
>>
>>
>>> Thus during the ext2fs_block_iterate2(), there should be some way to 
>>> tell if a block is preallocated or not.
>>
>> Unfortunately, the callback doesn't provide the extent flags. Unless,  
>> I miss something?
> 
> You're right, the iterator interface does not provide any extra info.
> 
> And I also checked the kernel implementation, they have extra 
> ext4_map_blocks() to do the resolve, and then ext4_es_lookup_extent() to 
> determine if it's unwritten.
> 
> So I'm afraid we have to go this solution.
> 
> 
> Meanwhile related to the implementation, can we put the prealloc flat 
> into blk_iterate_data?
> So that we can handle different fses' preallocated extents in a more 
> common way.
> 

I initially thought so, but is blk_iterate_data::num_blocks always
equal to extent::e_len in all file data extent situations mixed
with hole and unwritten combinations? If not, then the flag might
not be appropriate there, as it doesn't apply to the entirety of
blk_iterate_data::num_blocks.

Thanks, Anand

> Thanks,
> Qu