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SubjectRe: [RFC PATCH 0/2] Fix up the NFS mmap code
On Sat, Jan 09, 2010 at 01:38:25AM +0000, Al Viro wrote:
> On Fri, Jan 08, 2010 at 05:17:14PM -0800, Linus Torvalds wrote:
> >
> >
> > On Fri, 8 Jan 2010, Trond Myklebust wrote:
> > >
> > > How about something like the following. I chose to wrap the call to
> > > do_mmap_pgoff() instead of making a special ->pre_mmap(), since that
> > > seems more consistent with the way we handle ->read() and ->write().
> >
> > I still don't think that you can ever do mmap _and_ readdir on the same
> > inode, so there's something wrong with the lockdep annotations.
>
> readdir() is certainly a red herring. write(), OTOH, is quite real.
> And there we do i_mutex followed by pagefaults.
>
> I *REALLY* dislike Trond's solution, though.
>
> Could we please get a sane expalanation of the reasons why nfs mmap
> wants i_mutex in the first place? Before we add yet another hook
> from hell and complicate already overcomplicated area...

PS: mmap/write deadlock is real, AFAICT - unless something very subtle
prevents it, we can get buggered if we have two threads with the same
VM, mmap 1.4Mb from floppy and do
thread A:
write(fd_on_nfs, buffer_mmaped_from_floppy, 1440 * 1024);
thread B:
mmap(..., fd_on_nfs, ...)

It's not even particulary narrow.


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