Re: [RFC][PATCH] set_page_buffer_dirty should skip unmapped buffers

Jan Kara
2006-09-06T17:03:30Z

> On Wed, 2006-09-06 at 18:27 +0200, Jan Kara wrote:
> > >
> > > But my debug clearly shows that we are clearing the buffer, while
> > > we haven't actually submitted to ll_rw_block() code. (I added "track"
> > > flag to bh and set it in journal_commit_transaction() when we add
> > > them to wbuf[] and clear it in ll_rw_block() after submit. I checked
> > > for this flag in journal_unmap_buffer() while clearing the buffer).
> > > Here is what my debug shows:
> > >
> > > buffer is tracked bh ffff8101686ea850 size 1024
> > >
> > > Call Trace:
> > > [<ffffffff8020b395>] show_trace+0xb5/0x370
> > > [<ffffffff8020b665>] dump_stack+0x15/0x20
> > > [<ffffffff8030d474>] journal_invalidatepage+0x314/0x3b0
> > I see just journal_invalidatepage() here. That is fine. It calls
> > journal_unmap_buffer() which should do nothing return 0. If it does
> > not it would be IMO bug.. If the buffer is really unmapped here, in what
> > state it is (i.e. which list is it on?).
> >
> Acutally, I added dump_stack() in journal_unmap_buffer() when it
> does clear_buffer_mapped(). gcc must of pulled in the function ..
> I will add more debug to track the list bh came from.
Ah, ok. My guess is that the buffer is in BJ_Locked list. But we don't
write buffers from BJ_Locked list (as they are supposedly already
written). What probably happens is:
Process 1 Process 2
We start scanning t_sync_datalist.
Add buffer to wbuf[].
Locks the buffer (it can be some other
writer).
Restarts scanning the list
Finds buffer is locked -> moves to
BJ_Locked list (actually this is BUG
as we have not written the buffer yet)
journal_unmap_buffer() happens

Maybe what you could verify is, that we are moving the buffer added to
wbuf[] to BJ_Locked list in the write_out_data: loop.
If it really happens, what I've described above, the fix should be
different. We shouldn't have moved the buffer to BJ_Locked list...

Honza
--
Jan Kara <jack@suse.cz>
SuSE CR Labs