Re: [PATCH,preliminary] cleanup shm handling

[Christoph Rohland <cr@sap.com>]

David Howells <dhowells@redhat.com> writes:

> Can you help me with an SHM related problem?
> 
> I'm currently writing a Win32 emulation kernel module to help speed Wine up,
> and I'm writing the file mapping support stuff at the moment
> (CreateFileMapping and MapViewOfFile).

These two calls were exactly the reasons to make the shm fs for
linux. It makes posix shm possible which provides shm_open an
shm_unlink. This together with mmap and frtuncate gives you exactly
this functionality.

shm_open and shm_unlink are provided by glibc 2.2. If you don't want
to rely on that, you can use open ("/dev/shm/xxx") and hope that the
admin configured right.

> I have PE Image mapping just about working (fixups, misaligned file
> sections and all), but I'm trying to think of a good way of doing
> anonymous shared mappings without having to hack the main kernel
> around too much (so far I've only had to add to kernel/ksyms.c).
> 
> Is there a reasonable way I could hook into the SHM system to
> "reserve" a chunk of shared memory of a particular size, and then a
> second hook by which I can "map" _part_ of that into a process's
> address space?

Yes:

fd = shm_open ("xxx",...)
ptr = mmap (NULL, size, ..., fd, offset);

No kernel changes necessary on 2.4 any more.

Greetings
                Christoph

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