Re: [PATCH 0/5] Unify the return value of alloc/clone_extent_buffer()

[Qu Wenruo <wqu@suse.de>]

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On 2019/2/22 下午8:54, Nikolay Borisov wrote:
> 
> 
> On 22.02.19 г. 12:16 ч., Qu Wenruo wrote:
>> This patchset can be fetched from github:
>> https://github.com/adam900710/linux/tree/cleanup_alloc_extent_buffer
>> Which is based on v5.0-rc7
>>
>> There are 5 extent buffer alloc functions in btrfs:
>> __alloc_extent_buffer();
>> alloc_extent_buffer();
>> __alloc_dummy_extent_buffer();
>> alloc_dummy_extent_buffer();
>> alloc_test_extent_buffer();
>>
>> However their return value is not unified for failure mode:
>> __alloc_extent_buffer()		Never fail
>> alloc_extent_buffer()		PTR_ERR()
>> __alloc_dummy_extent_buffer()	NULL
> 
> This function can never return NULL, if __alloc_extent_buffer cannot
> fail then the only error this function returns is ERR_PTR(ENOMEM);

Nope.

        for (i = 0; i < num_pages; i++) {
                eb->pages[i] = alloc_page(GFP_NOFS);
                if (!eb->pages[i])
                        goto err; <<< Page alloc failure here
        }
...
err:
        for (; i > 0; i--)
                __free_page(eb->pages[i - 1]);
        __free_extent_buffer(eb);
        return NULL; << We got NULL.
}

For __alloc_dummy_extent_buffer, that's the only failure case.

And I'm interested how did you get the PTR_ERR() case?

> 
>> alloc_dummy_extent_buffer()	NULL
> Same thing applies to this function

Nope.

> 
>> alloc_test_extent_buffer()	NULL
> 
> Same thing for this function, if we return exists then we must have
> found it by find_extent_buffer hence it cannot be null. Otherwise we
> return eb as allocated from alloc_dummy_extent_buffer. So how can null
> be returned?

And nope.

> 
> To me it really seems none of the function could return a NULL value, no?

Your misunderstand of __alloc_dummy_extent_buffer() makes the call chain
all wrong.

Thanks,
Qu

> 
>>
>> This causes some wrapper function to have 2 failure modes, like
>> btrfs_find_create_tree_block() can return NULL or PTR_ERR(-ENOMEM) for
>> its failure.
>>
>> This inconsistent behavior is making static checker and reader crazy.
>>
>> This patchset will unify the failure more of above 5 functions to
>> PTR_ERR().
>>
>> Qu Wenruo (5):
>>   btrfs: extent_io: Add comment about the return value of
>>     alloc_extent_buffer()
>>   btrfs: extent_io: Unify the return value of __alloc_extent_buffer()
>>     with alloc_extent_buffer()
>>   btrfs: extent_io: Unify the return value of
>>     alloc_dummy_extent_buffer() with alloc_extent_buffer()
>>   btrfs: extent_io: Unify the return value of alloc_test_extent_buffer()
>>     with alloc_extent_buffer()
>>   btrfs: extent_io: Unify the return value of
>>     btrfs_clone_extent_buffer() with alloc_extent_buffer()
>>
>>  fs/btrfs/backref.c                     |  8 ++--
>>  fs/btrfs/ctree.c                       | 16 ++++----
>>  fs/btrfs/extent_io.c                   | 56 +++++++++++++++++---------
>>  fs/btrfs/qgroup.c                      |  5 ++-
>>  fs/btrfs/tests/extent-buffer-tests.c   |  6 ++-
>>  fs/btrfs/tests/extent-io-tests.c       |  4 +-
>>  fs/btrfs/tests/free-space-tree-tests.c |  3 +-
>>  fs/btrfs/tests/inode-tests.c           |  6 ++-
>>  fs/btrfs/tests/qgroup-tests.c          |  3 +-
>>  9 files changed, 68 insertions(+), 39 deletions(-)
>>
> 


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