Re: another nfs puzzle

[Peter Staubach <staubach@redhat.com>]

Trond Myklebust wrote:

>>Yup, same problem.  Why is this allowed?  Does it really work correctly?
>>    
>>
>
>Assuming that the processes have _some_ method of synchronisation, then
>I cannot see why it shouldn't be workable. Come to think of it, it might
>even be possible to use O_DIRECT to provide that synchronisation (use
>O_DIRECT to set a "lock" on the page, then modify it using mmap). 
>
>Whether or not there are people out there that actually _want_ to do
>this is a different matter.
>

Mixing O_DIRECT i/o and cached i/o is probably a recipe for disaster,
unless the cooperating programs are very careful and very aware of how
the particular file system in the particular kernel implements direct
i/o and caching, including cache validation.

This seems like a dangerous enough area that denying mmap on a file which
has been opened with O_DIRECT by any process and denying open(O_DIRECT)
on a file which has been mmap'd would be a good thing.  These things are
easy enough to keep track of, so it shouldn't be too hard to implement.

    Thanx...

       ps