FFS writes to read-only mount

[Pawel Jakub Dawidek]

On Wed, Mar 14, 2007 at 02:14:38PM +1000, David Cecil wrote:
> Hi,
> 
> I have seen the following message (or equivalent) occasionally on a FreeBSD 6.1 system:
> g_vfs_done():mirror/gm0s1a[WRITE(offset=1349091328, length=16384)]error = 1
> 
> The partition in question is the root partition, and it is mounted read-only.  I have verified that the problem occurs due to the write request returning EPERM due to the 
> check in g_io_check:
> case BIO_WRITE:
> case BIO_DELETE:
>        if (cp->acw == 0)
>            return (EPERM);
> 
> I have been trying to determine what within FFS would be trying to write to the partition.  The "bio_from" in the bio structure indicates (in the geom) that it's 
> ffs.mirror/gm0s1a that's trying to write.  The contents of the buffer looks somewhat like a directory (lots of files listed, but comparison to the actual directory that 
> contians these files reveals it's somewhat different), followed by a binary (ELF header).  However, I'm at a loss to understand who's actually doing the writing.  Is it 
> coming from within FFS or is there an application that's done the write?  (I can't understand how an application would be permitted to do it though.)
> 
> I have seen this sort of problem (same error number) reported on the Internet occasionally, but it doesn't seem it's been satisfactorily resolved in all instances.
> 
> Any help you can provide would be much appreciated.

Will be good if you could place kdb_enter() into g_vfs_done() error path
and once in DDB try which processes wait for I/O and collect their
backtraces or just put output of 'alltrace' on some web page.

-- 
Pawel Jakub Dawidek                       http://www.wheel.pl
pjd at FreeBSD.org                           http://www.FreeBSD.org
FreeBSD committer                         Am I Evil? Yes, I Am!
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