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Re: mmap(MAP_FIXED) vs mprotect

On 24/04/2013 8:50 AM, Corinna Vinschen wrote:
On Apr 24 08:03, Ryan Johnson wrote:
Hi all,

I'm trying to port a linux program that uses mmap to implement a
growable array; the ideas is to mmap(PROT_NONE, MAP_NORESERVE) a
chunk of address space (corresponding to the maximum array size) and
then call mmap(PROT_READ|PROT_WRITE, MAP_FIXED) to allocate actual
memory in the "blank" region. This works well in Linux but fails
with EINVAL in cygwin.
Oddly, trying to map in blank pages in with mprotect succeeds on
cygwin but fails with ENOMEM on linux...

Am I missing something here, or is this just a place where different
behavior between the two platforms is a fact of life? Which version
is the posixly "correct" way to reserve a chunk of address space and
later back it with actual memory?
There is no POSIXly correct way to do that since MAP_NORESERVE is a
non-POSIX extension in Linux as well as in Cygwin.

The general idea of MAP_NORESERVE is to make sure that we get as much
memory as requested, but to use only as much memory as is required.
On Linux MAP_NORESERVE only performs bookkeeping but doesn't change
the state of the memory, so a later mmap works.
Actually, MAP_NORESERVE is just to avoid getting OOM on machine with strict accounting if multiple 16GB spaces get allocated in this way; calling mmap with MAP_FIXED succeeds as long as the requested address range has already been allocated to the process in some way. In fact, the linux machine I tested on let me MAP_FIXED a 2MB region starting from inside a 16kB mapping, with the unfortunate side effect of clobbering the dynamic loader's state (which happened to be mapped nearby). That's a ridiculous amount of rope to hang yourself with, and I'm not confident it's possible, let alone desirable, to implement it in Cygwin (Windows refuses to unmap statically linked .dlls, for example). Still, the ability to clobber anonymous memory regions with some other mapping would be useful at times (e.g. to "concatenate" two files in memory by mapping them side by side). To be safe, we could require that region to be clobbered be PROT_NONE (which erases any data that may have been there previously, thus guaranteeing that we don't lose anything important by changing the mapping).
Having said that, we *could* also change mmap to handle this scenario
gracefully as well.
I guess hacking it in would be much less work than trying to improve fork() was, but realistically I'm not going to have time to do it any time soon (rats).


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