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, so this would be a problem for block_dev.c as well, then?
Because it would be possible to have a dirty block dev page
have its buffers reclaimed and then reinitialised via
init_page_buffers, AFAIKS.


> > I would have thought we can fix this simply by removing the
> > broken ramdisk_set_page_dirty (as far as the comment goes, we
> > set CAP_NO_ACCT_DIRTY anyway, so the normal set_page_dirty
> > should handle everything properly, no?).
>
>
> No.  I don't know where accounting comes into play.  I didn't
> trace that path.  But if we have a non-dirty ramdisk page with
> buffers (basically a hole in the middle or at the end of the ramdisk).
> We need to set the buffer dirty bits when we write to it.

Accounting is done in set_page_dirty.

> So I don't see how it would make sense to reuse the generic
> set_page_dirty, and handling all of the logic in set_page_dirty
> to dirty the buffer heads seemed to have made the most sense.

That's what the generic set_page_dirty does. What I want to know
is why *doesn't* it make sense to reuse the generic set_page_dirty?
Unless there is a good reason, then reusing is better than writing
your own.

--
SUSE Labs, Novell Inc.
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