Re: [PATCH -next v2 03/22] ext4: only order data when partially block truncating down

[Zhang Yi]

On 2/11/2026 7:42 PM, Jan Kara wrote:
> On Wed 11-02-26 00:11:51, Zhang Yi wrote:
> > On 2/10/2026 10:07 PM, Jan Kara wrote:
> > > On Tue 10-02-26 20:02:51, Zhang Yi wrote:
> > > > On 2/9/2026 4:28 PM, Zhang Yi wrote:
> > > > > On 2/6/2026 11:35 PM, Jan Kara wrote:
> > > > > > On Fri 06-02-26 19:09:53, Zhang Yi wrote:
> > > > > > > On 2/5/2026 11:05 PM, Jan Kara wrote:
> > > > > > > > So how about the following:
> > > > > > >
> > > > > > > Let me see, please correct me if my understanding is wrong, ana there are
> > > > > > > also some points I don't get.
> > > > > > >
> > > > > > > > We expand our io_end processing with the
> > > > > > > > ability to journal i_disksize updates after page writeback completes. Then
> > > >
> > > > While I was extending the end_io path of buffered_head to support updating
> > > > i_disksize, I found another problem that requires discussion.
> > > >
> > > > Supporting updates to i_disksize in end_io requires starting a handle, which
> > > > conflicts with the data=ordered mode because folios written back through the
> > > > journal process cannot initiate any handles; otherwise, this may lead to a
> > > > deadlock. This limitation does not affect the iomap path, as it does not use
> > > > the data=ordered mode at all.  However, in the buffered_head path, online
> > > > defragmentation (if this change works, it should be the last user) still uses
> > > > the data=ordered mode.
> > >
> > > Right and my intention was to use reserved handle for the i_disksize update
> > > similarly as we currently use reserved handle for unwritten extent
> > > conversion after page writeback is done.
> >
> > IIUC, reserved handle only works for ext4_jbd2_inode_add_wait(). It doesn't
> > work for ext4_jbd2_inode_add_write() because writebacks triggered by the
> > journaling process cannot initiate any handles, including reserved handles.
>
> Yes, we cannot start any new handles (reserved or not) from writeback
> happening from jbd2 thread. I didn't think about that case so good catch.
> So we can either do this once we have delay map and get rid of data=ordered
> mode altogether or, as you write below, we have to submit the tail folios
> proactively during truncate up / append write - but I don't like this
> option too much because workloads appending to file by small chunks (say a
> few bytes) will get a large performance hit from this.

Yeah, so let's keep the buffered_head path as it is now, and only modify 
the iomap path to support the new post-EOF block zeroing solution for 
truncate up and append write as discussed.

Cheers,
Yi.

> > So, I guess you're suggesting that within mext_move_extent(), we should
> > proactively submit the blocks after swapping, and then call
> > ext4_jbd2_inode_add_wait() to replace the existing
> > ext4_jbd2_inode_add_write(). Is that correct?
>
> 								Honza