In case I'm not clear: I'm talking about read-only, MAP_DENYWRITE
mappings. There's no coherency issue for these.
> ... and this simply means better performance.
Disallowing non-aligned read-only mappings means worse performance for
Wine. Windows binaries want to be mapped without page alignment.
Windows binaries can be quite large: I've seen Wine read in
approx. 100MB of library code because it couldn't mmap, just to launch
an application. It would be good to demand page these. It would be
good to share the loaded images just like any other executable or shared
library.
It's possible to do demand paging, in principle, using SIGSEGVs for
user-space demand paging. Maybe Wine even does this now (I remember it
being discussed and I haven't followed development in detail). But
that's unable to demand *unmap* like a kernel mapping: only the kernel
knows when those pages should be used for something else. It has other
inefficiencies too.
I have considered writing some kernel code to do non-aligned, read-only,
MAP_DENYWRITE mappings "on top" of existing mappings. I.e. the
non-aligned mapping would be demand page just like any other, and work
by _nopage simply copying the data from the page cache. This is
hopefully nice and clean because it doesn't affect the normal mapping
code at all. Is there any interest in such code?
-- Jamie
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