Re: [PATCH 02/20] ext4: prevent partial update of the extents path

[Baokun Li]

Hi Ojaswin,

On 2024/7/16 17:54, Ojaswin Mujoo wrote:
> > > But the journal will ensure the consistency of the extents path after
> > > this patch.
> > >
> > > When ext4_ext_get_access() or ext4_ext_dirty() returns an error in
> > > ext4_ext_rm_idx() and ext4_ext_correct_indexes(), this may cause
> > > the extents tree to be inconsistent. But the inconsistency just
> > > exists in memory and doesn't land on disk.
> > >
> > > For ext4_ext_get_access(), the handle must have been aborted
> > > when it returned an error, as follows:
> > ext4_ext_get_access
> >   ext4_journal_get_write_access
> >    __ext4_journal_get_write_access
> >     err = jbd2_journal_get_write_access
> >     if (err)
> >       ext4_journal_abort_handle
> > > For ext4_ext_dirty(), since path->p_bh must not be null and handle
> > > must be valid, handle is aborted anyway when an error is returned:
> > ext4_ext_dirty
> >   __ext4_ext_dirty
> >    if (path->p_bh)
> >      __ext4_handle_dirty_metadata
> >       if (ext4_handle_valid(handle))
> >         err = jbd2_journal_dirty_metadata
> >          if (!is_handle_aborted(handle) && WARN_ON_ONCE(err))
> >            ext4_journal_abort_handle
> > > Thus the extents tree will only be inconsistent in memory, so only
> > > the verified bit of the modified buffer needs to be cleared to avoid
> > > these inconsistent data being used in memory.
> > Regards,
> > Baokun
> Thanks for the explanation Baokun, so basically we only have the
> inconsitency in the memory.
>
> I do have a followup questions:
>
> So in the above example, after we have the error, we'll have the buffer
> for depth=0 marked as valid but pointing to the wrong ei_block.
It looks wrong here. When there is an error, the ei_block of the
unmodified buffer with depth=0 is the correct one, it is indeed
'valid' and it is consistent with the disk. Only buffers that were
modified during the error process need to be checked.

Regards,
Baokun
> In this case, can we have something like below:
>
> -----------------
> ext4_ext_remove_space
>    err = ext4_ext_rm_idx (error, path[0].p_bh inconsistent but verified)
>    /*
>     * we release buffers of the path but path[0].p_bh is not cleaned up
>     * due to other references to it (possible?)
>     */
>
> ... at a later point...:
>
> ext4_find_extent
>    bh = read_extent_tree_block()
>      /*
>       * we get the bh that was left inconsistent previously
>       * since its verified, we dont check it again corrupting
>       * the lookup
>       */
>
> -----------------
>
> Is the above scenario possible? Or would the path[0].p_bh that was
> corrupted previously always be reread during the subsequent
> ext4_find_extent() lookup?
>
> Thanks again,
> Ojaswin