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SubjectRe: [PATCH] ext4: Fix i_disksize exceeding i_size problem in paritally written case
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>> BTW, I want send another patch as follows:
>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
>> index bf0b7dea4900..570a687ae847 100644
>> --- a/fs/ext4/inode.c
>> +++ b/fs/ext4/inode.c
>> @@ -3149,7 +3149,7 @@ static int ext4_da_write_end(struct file *file,
>> return ext4_write_inline_data_end(inode, pos, len, copied,
>> page);
>>
>> start = pos & (PAGE_SIZE - 1);
>> - end = start + copied - 1;
>> + end = start + (copied ? copied - 1 : copied);
>>
>> /*
>> * Since we are holding inode lock, we are sure i_disksize <=
>> @@ -3167,7 +3167,7 @@ static int ext4_da_write_end(struct file *file,
>> * ext4_da_write_inline_data_end().
>> */
>> new_i_size = pos + copied;
>> - if (copied && new_i_size > inode->i_size &&
>> + if (new_i_size > inode->i_size &&
>> ext4_da_should_update_i_disksize(page, end))
>> ext4_update_i_disksize(inode, new_i_size);
>>
>> This modification handle unconsistent i_size and i_disksize imported by
>> ea51d132dbf9 ("ext4: avoid hangs in ext4_da_should_update_i_disksize()").
>>
>> Paritially written may display a fake inode size for user, for example:
>>
>>
>>
>> i_disksize=1
>>
>> generic_perform_write
>>
>> copied = iov_iter_copy_from_user_atomic(len) // copied = 0
>>
>> ext4_da_write_end // skip updating i_disksize
>>
>> generic_write_end
>>
>> if (pos + copied > inode->i_size) { // 10 + 0 > 1, true
>>
>> i_size_write(inode, pos + copied); // i_size = 10
>>
>> }
>>
>>
>>
>> 0 1 10 4096
>>
>> |_|_______|_________..._____|
>>
>> | |
>>
>> i_size pos
>>
>>
>>
>> Now, user see the i_size is 10 (i_disksize is still 1). After inode
>>
>> destroyed, user will get the i_size is 1 read from disk.
>
> OK, but shouldn't we rather change generic_write_end() to not increase
> i_size if no write happened? Because that is what seems somewhat
> problematic...
>
> Honza
>

After looking through some code, I find some other places have similar
problem:
1. In ext4_write_end(), i_size is updated by ext4 not generic_write_end().
2. The iommap framework, i_size is updated even copied is zero.
3. ubifs_write_end, i_size is updated even copied is zero.

It seems that fixing all places is not an easy work.

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