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

[Baokun Li]

On 2024/7/17 13:29, Ojaswin Mujoo wrote:
> On Tue, Jul 16, 2024 at 07:54:43PM +0800, Baokun Li wrote:
> > 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
> Hey Baokun,
>
> Ahh I see now, I was looking at it the wrong way. So basically since
> depth 1 to 4 is inconsistent to the disk we mark then non verified so
> then subsequent lookups can act accordingly.
>
> Thanks for the explanation! I am in the middle of testing this patchset
> with xfstests on a POWERPC system with 64k page size. I'll let you know
> how that goes!
>
> Regards,
> Ojaswin

Hi Ojaswin,

Thank you for the test and feedback!

Cheers,
Baokun