Re: [patch] mm: bug in set_page_dirty_buffers

[Andrew Morton <akpm@osdl.org>]

On Tue, 10 Oct 2006 10:18:20 +0200
Nick Piggin <npiggin@suse.de> wrote:

> > If the buffer's redirtied after discard_buffer() got at it, we've got some
> > nasty problems in there.
> > 
> > Are you sure this race can happen?  Nobody's allowed to have a page mapped
> > while it's undergoing truncation (vmtruncate()).
> 
> Well, not technically. I think it can happen with nonlinear pages now,

How?

> but that is a bug in truncate and I have some patches to fix them.
> 
> But anyone who has done a get_user_pages, AFAIKS, can later run a
> set_page_dirty on the pages.

Most (all?) callers are (and should be) using set_page_dirty_lock().

> Nasty problem, but I think we are OK to
> just ignore the dirty bits in this case, because the truncate might
> well have happened _after_ the get_user_pages guy set the page dirty
> anyway.
> 
> > There might be a problem with the final blocks in the page outside i_size. 
> > iirc what happens here is that the bh outside i_size _is_ marked dirty, but
> > writepage() will notice that it's outside i_size and will just mark it
> > clean again without doing IO.
> 
> Didn't think of that. How does it get to writepage though, if it has
> lost its mapping?

It was only partially truncated - it's still on the address_space.

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