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

[Nikolay Borisov <n.borisov.lkml@gmail.com>]

On 22.02.19 г. 15:02 ч., Qu Wenruo wrote:
> 
> 
> 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.

Right, I was looking at the code AFTER having applied your patches. So I
agree with yout. However, the ordering of your patches and the
changelogs make it rather hard to understand. What I'd suggest regarding
the changelogs is - forget about unification, just say what you are
doing, which is always ensuring that an error is returned from
__alloc_extent_buffer in one patch - this should involve both changes to
__alloc_extent_buffer as well as it's (in)direct callers. Then you do
the same for other function. Otherwise review is somewhat hindered.

> }
> 
> 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(-)
>>>
>>
>