Re: mmap/munmap semantics

[James Antill <james@and.org>]

> Hi!
> 
> With the ongoing development of GLAME there arise the following
> problems with the backing-store management, which is a mmaped
> file and does "userspace virtual memory management":
> - I cannot see a way to mmap a part of the file but set the
>   contents initially to zero, i.e. I want to setup an initially
>   dirty zero-mapping which is assigned to a part of the file.
>   Currently I'm just mmaping the part and do the zeroing by
>   reading from /dev/zero (which does as I understand from the
>   kernel code just create this zero mappings) - is there a more
>   portable way to achieve this?

 I think you want to truncate/lseek after open but before you mmap,
if I'm reading what you want to do properly.
 This is portable (at least it works on Linux/FreeBSD/Solaris).

> - I need to "drop" a mapping sometimes without writing the contents
>   back to disk - I cannot see a way to do this with linux currently.
>   Ideally a hole could be created in the mmapped file on drop time -
>   is this possible at all with the VFS/ext2 at the moment (creating
>   a hole in a file by dropping parts of it)?

 The mapping can be synchronized with the file before you munmap() so
I'm not sure what you want mrevert() ? -- I'm positive you're going to
have to do this in user space (Ie. copy the file and then rename() or
don't rename() at munmap() time -- or do a private mapping and write()
to the file at munmap()).

-- 
James Antill -- james@and.org
I am always an optimist, but frankly there is no hope.
   -Hosni Mubarek
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