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

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Message-ID: <ZqCd0fjFzZt00h6N@li-bb2b2a4c-3307-11b2-a85c-8fa5c3a69313.ibm.com>
Date: Wed, 24 Jul 2024 11:53:13 +0530
From: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To: Baokun Li <libaokun@...weicloud.com>
Cc: linux-ext4@...r.kernel.org, tytso@....edu, adilger.kernel@...ger.ca,
        jack@...e.cz, ritesh.list@...il.com, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com, yangerkun@...wei.com,
        Baokun Li <libaokun1@...wei.com>,
        zhanchengbin <zhanchengbin1@...wei.com>
Subject: Re: [PATCH 02/20] ext4: prevent partial update of the extents path

On Wed, Jul 17, 2024 at 02:11:27PM +0800, Baokun Li wrote:
> 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

Hey Baokun,

The xfstests pass for sub page size as well as bs = page size for
POWERPC with no new regressions.

Although for this particular patch I doubt if we would be able to
exersice the error path using xfstests. We might need to artifically 
inject error in ext4_ext_get_access or ext4_ext_dirty.  Do you have any
other way of testing this? 

Also, just curious whether you came across this bug during code reading
or were you actually hitting it?

Regards,
Ojaswin
>