linux-ext4 - Re: [PATCH v4 3/8] ext4: introduce new callback for IOMAP

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Message-Id: <20191009131441.BDE6311C054@d06av25.portsmouth.uk.ibm.com>
Date:   Wed, 9 Oct 2019 18:44:40 +0530
From:   Ritesh Harjani <riteshh@...ux.ibm.com>
To:     Matthew Bobrowski <mbobrowski@...browski.org>
Cc:     tytso@....edu, jack@...e.cz, adilger.kernel@...ger.ca,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        hch@...radead.org, david@...morbit.com, darrick.wong@...cle.com
Subject: Re: [PATCH v4 3/8] ext4: introduce new callback for IOMAP_REPORT
 operations



On 10/9/19 5:38 PM, Matthew Bobrowski wrote:
> On Wed, Oct 09, 2019 at 11:30:21AM +0530, Ritesh Harjani wrote:
>>> +static u16 ext4_iomap_check_delalloc(struct inode *inode,
>>> +				     struct ext4_map_blocks *map)
>>> +{
>>> +	struct extent_status es;
>>> +	ext4_lblk_t end = map->m_lblk + map->m_len - 1;
>>> +
>>> +	ext4_es_find_extent_range(inode, &ext4_es_is_delayed, map->m_lblk,
>>> +				  end, &es);
>>> +
>>> +	/* Entire range is a hole */
>>> +	if (!es.es_len || es.es_lblk > end)
>>> +		return IOMAP_HOLE;
>>> +	if (es.es_lblk <= map->m_lblk) {
>>> +		ext4_lblk_t offset = 0;
>>> +
>>> +		if (es.es_lblk < map->m_lblk)
>>> +			offset = map->m_lblk - es.es_lblk;
>>> +		map->m_lblk = es.es_lblk + offset;
>> This looks redundant no? map->m_lblk never changes actually.
>> So this is not needed here.
> 
> Well, it depends if map->m_lblk == es.es_lblk + offset prior to the
> assignment? If that's always true, then sure, it'd be redundant. But
> honestly, I don't know what the downstream effect would be if this was
> removed. I'd have to look at the code, perform some tests, and figure
> it out.

<snip>
3334         if (es.es_lblk <= map->m_lblk) {
3335                 ext4_lblk_t offset = 0;
3336
3337                 if (es.es_lblk < map->m_lblk)
3338                         offset = map->m_lblk - es.es_lblk;
3339                 map->m_lblk = es.es_lblk + offset;
3340                 map->m_len = es.es_len - offset;
3341                 return IOMAP_DELALLOC;
3342         }

I saw it this way-

In condition "if (es.es_lblk <= map->m_lblk)" there are 2 cases.

Case 1: es.es_lblk is equal to map->m_lblk (equality)
    For this case, "offset" will remain 0.
    So map->lblk = es.es_lblk + 0 (but since es.es_lblk is same as
map->m_lblk in equality case, so it is redundant).


Case 2: es.es_lblk < map->m_lblk (less than)
In this case "offset = map->m_lblk - es.es_lblk"
Now replacing this val of offset in "map->m_lblk = es.es_lblk + offset"
map->m_lblk = es.es_lblk + map->m_lblk - es.es_lblk
which again is map->m_lblk = map->m_lblk - again redundant.


What did I miss?
But sure feel free to test as per your convenience.


> 
>>> +	map.m_lblk = first_block;
>>> +	map.m_len = last_block = first_block + 1;
>>> +	ret = ext4_map_blocks(NULL, inode, &map, 0);
>>> +	if (ret < 0)
>>> +		return ret;
>>> +	if (ret == 0)
>>> +		type = ext4_iomap_check_delalloc(inode, &map);
>>> +	return ext4_set_iomap(inode, iomap, type, first_block, &map);
>> We don't need to send first_block here. Since map->m_lblk
>> is same as first_block.
>> Also with Jan comment, we don't even need 'type' parameter.
>> Then we should be able to rename the function
>> ext4_set_iomap ==> ext4_map_to_iomap. This better reflects what it is
>> doing. Thoughts?
> 
> Depends on what we conclude in 1/8. :)
> 
> I'm for removing 'first_block', but still not convinced removing
> 'type' is heading down the right track if I were to forward think a
> little.

Only once you are convinced that map->m_lblk will not change even in
function ext4_iomap_check_delalloc(), then only you should
drop "first_block" argument from ext4_set_iomap.

Please check above comments once.

-ritesh