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
