Hello!

> The struct file that is used appears impossible for user space 
> to get at directly.  Therefore I believe we can instead increment
> and decrement the namespace count at the same places we increment
> and decrement shm_nattach.  Ideally we would only increment the
> namespace count when shm_nattach goes from 0 to 1 and we would
> only decrement the namespace count when shm_nattach goes from 1 to 0.
> 
> Does that make sense?

Yes, this would save the day.

Indeed, shm_file_ns() is required only when the segment is already mapped,
except for shm_mmap() and even there shm_nattch is incremented before
do_mmap() is used. It will work.

Possibility to use this file directly will be lost. It is a little unpleasant;
openvz checkpointing used it to restore sysv shm mappings like another file
mappings, it was nice, but this code can be a little uglified to treat
those mapping specially. No harm either.

Alexey
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