Re: [patch 2/2] fs: fix page_mkwrite error cases in core code and btrfs

[Trond Myklebust <trond.myklebust@fys.uio.no>]

On Wed, 2009-03-11 at 04:55 +0100, Nick Piggin wrote:
> page_mkwrite is called with neither the page lock nor the ptl held. This
> means a page can be concurrently truncated or invalidated out from underneath
> it. Callers are supposed to prevent truncate races themselves, however
> previously the only thing they can do in case they hit one is to raise a
> SIGBUS. A sigbus is wrong for the case that the page has been invalidated
> or truncated within i_size (eg. hole punched). Callers may also have to
> perform memory allocations in this path, where again, SIGBUS would be wrong.
> 
> The previous patch made it possible to properly specify errors. Convert
> the generic buffer.c code and btrfs to return sane error values
> (in the case of page removed from pagecache, VM_FAULT_NOPAGE will cause the
> fault handler to exit without doing anything, and the fault will be retried 
> properly).
> 
> This fixes core code, and converts btrfs as a template/example. All other
> filesystems defining their own page_mkwrite should be fixed in a similar
> manner.

There appears to be another atomicity problem in the same area of
code...

The lack of locking between the call to ->page_mkwrite() and the
subsequent call to set_page_dirty_balance() means that the filesystem
may actually already have written out the page by the time you get round
to calling set_page_dirty_balance().

How then is the filesystem supposed to guarantee that whatever structure
it allocated in page_mkwrite() is still around when the page gets marked
as dirty a second time?

Trond

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