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SubjectRe: [PATCH] ext4: Fix i_disksize exceeding i_size problem in paritally written case
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> On Mon 20-03-23 20:49:07, Zhihao Cheng wrote:
>> [...]
>>
>>>> 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.
>
> Well, yeah, probably not trivial but still desirable ;). Will you look into
> that?
>
> Honza
>


I'am happy to investigate it, maybe it will take some time, and I'm also
glad to help review code if somebody come up a solution firstly.

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