Re: [PATCH 1/3] Preserve the dirty bit in init_page_buffers

[Nick Piggin <nickpiggin@yahoo.com.au>]

Eric W. Biederman wrote:
> Nick Piggin <nickpiggin@yahoo.com.au> writes:
>
>
> > Eric W. Biederman wrote:
> >
> > > The problem:  When we are trying to free buffers try_to_free_buffers
> > > will look at ramdisk pages with clean buffer heads and remove the
> > > dirty bit from the page.  Resulting in ramdisk pages with data that
> > > get removed from the page cache.  Ouch!
> > >
> > > Buffer heads appear on ramdisk pages when a filesystem calls getblk,
> > > which through a series of function calls eventually calls
> > > init_page_buffers.
> > >
> > > So to fix the mismatch between buffer head state and page state this
> > > patch modifies init_page_buffers to transfer the dirty bit from the
> > > page to the buffer heads like we currently do for the uptodate bit.
> >
> > Ouch indeed!
> >
> > But can we ever have a dirty page at init_page_buffers-time?
>
>
> Definitely, and it was a royal pain to trace the bug that this
> caused.  An initial ramdisk having pieces disappear after mkfs
> is called can look like the entire machine is dying.
>
> When we initialize the ramdisk by writing to /dev/ram0 usually in
> init/do_mounts_rd.c we don't allocate buffer heads but we do set
> the dirty bit, and the page is in the page cache.  So when we
> later call getblk it reuses the same page and then calls
> init_page_buffers.

Hmm, the comment above grow_dev_buffers indicates this should
not happen. But contrary to the comment, it doesn't go BUG
unless you're attaching dirty buffers to a page with dirty
buffers.

I suspect this happens more frequently with rd.c, because unlike
block_dev.c, it does not create dirty buffers in prepare_write.
However it could still happen in block_dev.c via mmaped memory,
as I said earlier.

I'm not saying this patch 1/3 is wrong, but we would at least
need to revise some comments. The grow_dev_buffers comment looks
like one of Andrew's. Maybe he can shed some more light on this?

--
SUSE Labs, Novell Inc.
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