Re: [PATCH] ext3: ext3_commit_super should always mark super uptodate

[Jeff Mahoney <jeffm@suse.com>]

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Phillip Susi wrote:
> Jeff Mahoney wrote:
>>  After a superblock write failure, the buffer_head is marked !uptodate.
>>  Since the superblock is something of an exception -- read once and
>>  a reference kept for the duration of the mount -- it is by definition
>>  always uptodate.
> 
> Not if it has been modified and not yet written back to disk.  Then it
> is dirty by definition.

I think you're missing the distinction between uptodate and dirty here.

>>  This is somewhat academic for the most part until we encounter error
>>  conditions. For example, if a disk goes away in a SAN environment, the
>>  write failure will occur and it will be followed by others. ext3_error
>>  wants to mark the superblock dirty via ext3_commit_super, but once
>>  the first write fails, the subsequent mark_buffer_dirty calls will
>>  issue warnings because the buffer is not uptodate.
> 
> Why on earth is a warning issued for marking an already dirty buffer
> dirty again?  Surly buffers can be modified again and thus, marked as
> dirty a second time before pdflush gets around to syncing them all the
> time.

No, it issues a warning when you mark a !uptodate buffer dirty. The idea
behind it is that the buffer contents may be stale with respect to disk
contents. In the case of the superblock, the in-memory contents are
always the most recent and it *never* re-reads it from disk.

>> +    set_buffer_uptodate(sbh);
>>      mark_buffer_dirty(sbh);
>>      if (sync)
>>          sync_dirty_buffer(sbh);
> 
> Are you sure this only matters when there is an IO error?  It looks
> there like if !sync then the sb can continue to sit in memory marked as
> dirty for some time before someone tries to flush it to disk. Eventually
> can't we end up back through this code patch and calling
> mark_buffer_dirty again before it was ever flushed in the first place,
> even without an IO error?

Again, uptodate != dirty.

- -Jeff

- --
Jeff Mahoney
SUSE Labs
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