Re: [PATCH] ext4: Fix i_disksize exceeding i_size problem in paritally written case

[Zhihao Cheng]

[...]

> > 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.