FFS writes to read-only mount

[Pawel Jakub Dawidek]

On Thu, Mar 15, 2007 at 11:00:31AM +0200, Kostik Belousov wrote:
> On Thu, Mar 15, 2007 at 04:56:39PM +1000, David Cecil wrote:
> > Hi Pawel,
> > 
> > here is what I've found.  Note that this problem doesn't occur 
> > frequently, but does happen often enough to be a problem.  I am still 
> > debugging the live system.
> > 
> > I put breakpoints in g_print_bio and g_io_request.  I then continued 
> > until the breakpoint at g_print_bio breakpoint was hit, but each time 
> > g_io_request was hit, I printed the backtrace.  Then, at g_print_bio I 
> > could identify the struct bio * and look for the corresponding 
> > g_io_request trace with a matching struct bio *.
> > 
> > That trace looks like this:
> > db> bt
> > Tracing pid 47 tid 100050 td 0x860d0640
> > g_io_request(864258c4,860ec440,a0020024,98b92188,a5281bf0,...) at 
> > g_io_request+0x107
> > g_vfs_strategy(86104ce4,98b92188,a0020024,98b92188,86104c30,...) at 
> > g_vfs_strategy+0x63
> > ffs_geom_strategy(86104ce4,98b92188) at ffs_geom_strategy+0x129
> > bufwrite(98b92188,4000,98b92188,a5281c54,805b20c1,...) at bufwrite+0x146
> > ffs_bufwrite(98b92188) at ffs_bufwrite+0x282
> > vfs_bio_awrite(98b92188) at vfs_bio_awrite+0x221
> > vop_stdfsync(a5281cbc,a5281cbc,a5281c98,806eff3e,a5281cbc,...) at 
> > vop_stdfsync+0x121
> It would be helpful, at this frame, to print the actual vnode, as well as
> the dirty buffer header together with dump of buffer data. [...]

This comes from devfs_fsync(), so I suspect it's devvp, and we are
synchronizing entire file system here.

> [...] It seems that
> the easiest way to do that would be to obtain coredump and then use kgdb.

On successfull remount to read-only we could walk (under DIAGNOSTIC) the
list of vnodes and assert there are no dirty buffers, but I don't think
this is the case...
The thing that worries me is that it doesn't happen from what I
understand just after remount to read-only. Syncer wakeups every 30
seconds by default, right? So it should generates errors on first time
he synchronizes file system. Which bascially means, that something tries
to write to read-only file system later. Will be good to assert it
somehow earlier. One thing that comes to my mind is to assert:

	KASSERT(((struct g_consumer *)buf->b_bufobj.bo_private)->acw > 0);

in each b*write() function. This is probably not the only way to write
to a file system...

I still don't feel too familiar with the code, so not sure if my
comments are usable:) how-to-reproduce procedure would be the best of
course... David, do you use something which can be considered
non-standard, like snapshots?

-- 
Pawel Jakub Dawidek                       http://www.wheel.pl
pjd at FreeBSD.org                           http://www.FreeBSD.org
FreeBSD committer                         Am I Evil? Yes, I Am!
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 187 bytes
Desc: not available
Url : http://lists.freebsd.org/pipermail/freebsd-fs/attachments/20070315/ba7b8a82/attachment.pgp