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

[Sage Weil <sage@newdream.net>]

On Fri, 13 Mar 2009, Nick Piggin wrote:
> On Thu, Mar 12, 2009 at 04:03:57PM -0700, Sage Weil wrote:
> > On Thu, 12 Mar 2009, Trond Myklebust wrote:
> > > 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().
> > 
> > We were just banging our heads against this issue last week.
> 
> That's coming too:
> http://marc.info/?l=linux-fsdevel&m=123555461816471&w=2
> 
> (we ended up deciding to call with page unlocked and return with locked,
> as it solves locking problems in some filesystems).
> 
> I'll resend that patch soonish. Hopefully it will work for you two?

Yeah, that'll work nicely.

Thanks!
sage

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