Re: (buggy) patch to get mmap() working on nfs.

[Thomas Bushnell, BSG]

Marcus Brinkmann <Marcus.Brinkmann@ruhr-uni-bochum.de> writes:

> I have this buggy patch here to get mmap() working on nfs.
> Simply apply it, compile nfs and try to mmap a file from it.  For me, the
> computer locks solid :) but hey, your mileage may vary.  I don't know how
> much time I will have to debug this in the next time, so someone please beat
> me to it!  (I did not really wrote any of that code, so it's as hard for you
> as it is for me).  The netfs_S_io_map implementation is ripped from
> libdiskfs, pager.c was in nfs/ but never used.

Um, I'm pretty sure (even without looking at it) that your implication
is buggy.

You must promise the kernel that *any* change to the underlying data
for the mapped object will never change except with you telling the
kernel.  There is no way to do that on NFS, because the remote file
could get changed underneath you.

The only way to implement mmap correctly is to transfer the entire
file over the wire and keep a local copy before you serve any pages.