Troy Rollo wrote:
It has a few problems though. Firstly, we'd miss mmaps done with system
calls.
Yes, but how many apps actually do this?
We wouldn't care about applications, only libraries that use their own
mmap system call (eg. Wine).
Secondly, we'd have to make assumptions about what areas of
memory the kernel would let us map, and what areas of memory were
already used in the address map.
It is possible to discover this by attempting to map all areas. It would not
need to be done page-by-page because we could use something akin to a binary
search (actually if we really wanted to be rude about it we could grovel
at /proc/self/maps, which would work (absent a debugger), but is non-portable
- of course this whole technique is non-portable).
If the system call fails because part of the address space it was attempting
to allocate was already used, the same technique could be used to discover
what was mapped and mark that space as temporarily "poisoned" (if we later
run out of address space, or perhaps at the time we free neighbouring
addresses, we could attempt to reclaim it).
Well, what's needed is a proof of concept patch so that we can explore
the issues with it, show the advantages and submit it to Alexandre to
finally be rejected :)
Aren't kernel modules considered a last resort? Would it even win anything
given that we still call native libraries which are then likely to call
libc's mmap (directly or indirectly)?
I didn't write it to solve this problem in particular... but having the
kernel know about Wine's special memory layout requirements is probably
the only way to solve the problem once and for all.
Mike
