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
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
