Re: [patch] mm: bug in set_page_dirty_buffers

[Andrew Morton <akpm@osdl.org>]

On Tue, 10 Oct 2006 07:22:48 +0200
Nick Piggin <npiggin@suse.de> wrote:

> On Mon, Oct 09, 2006 at 10:01:27PM -0700, Andrew Morton wrote:
> > On Tue, 10 Oct 2006 06:47:45 +0200
> > Nick Piggin <npiggin@suse.de> wrote:
> > > > There we can trylock all the pages and bale if any fail.
> > > 
> > > Hmm, try_to_unmap is OK because the page is already locked. page_remove_rmap
> > > isn't allowed to fail.
> > 
> > I was talking about try_to_unmap_cluster().
> 
> But page_remove_rmap's many callers are still screwed. Take do_wp_page,
> for example.
> 
> > > > But where?  locking the page is the preferred way to solve this stuff. 
> > > > (Well, locking the buffers might work, but isn't needed, and locking the
> > > > page covers other stuff)
> > > 
> > > AFAIKS, it is just fs/buffer.c that is racy.
> > 
> > Need to review all ->set_page_dirty, ->writepage, ->invalidatepage, ->etc
> > implementations before we can say that.

    ^^^ this

> > > Why can't it use
> > > mapping->private_lock or the buffer bit spinlock?
> > 
> > block_invalidatepage() wants to do lock_buffer().
> > 
> > It can probably be made to work.  But a sane interface is "when dinking
> > with page internals, lock the page".
> 
> I disagree because it will lead to horrible hacks because many callers
> can't sleep. If anything I would much prefer an innermost-spinlock in
> page->flags that specifically excludes truncate. Actually tree_lock can
> do that now, provided we pin mapping in all callers to set_page_dirty
> (which we should do).
> 
> Then the locking protocol is up to fs/buffer.c. You could set a bit in
> the buffer "BH_Invalidated" in truncate before clearing dirty, and test
> for that bit in set_page_dirty_buffers?

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