This is the mail archive of the
mailing list for the GCC project.
Re: Mmap tests on the Hurd
On Sun, Jan 21, 2001 at 11:46:44PM +0100, Mark Kettenis wrote:
> > As a sidenote, the fact that the autoconf tests die with a fatal
> > signal is a bit annoying, since the Hurd's default behaviour is to
> > suspend the program instead of dumping core (such that you can
> > attach the debugger to it) which means that the whole configuration
> > process is suspended.
> Is there a way to override this for just the test process? We don't
> need a core dump, just a forcible process exit.
> In principle there is. From looking at the code setting the
> environment variable CRASHSERVER to /dev/null should work, but I
> haven't verified this. But if SIGBUS is trapped in the same way as
> SIGSEGV, there is no real need to infect the test program with
> Hurd-specific mumbo-jumbo.
I think it might be wise to infect the entire configure script with
this particular mumbo jumbo, as you put it. What if some other test
process gets a fatal signal? The naive user will see configure hang
with no explanation. [This should really be dealt with by the
autoconf maintainers, but gcc's got plenty of local hacks already...]
> > 2. The Hurd is "so perverse as to leave unmapped space between
> > consecutive calls to mmap" :-). I'm not exactly sure why. It can
> > probably be blamed on Mach, or there might be a "bug" in the Hurd's
> > mmap implementation here. Do you really need this behaviour?
> Could you please instrument anonmap() in the test so it prints the
> address and size of each allocation, then run the test again, and
> examine the results carefully? If addresses allocated in the first
> two tests are not being recycled, that's a serious problem.
> OK, thanks for the explanation. I did some further investigation, and
> things turn out to be ... interesting. The addresses are properly
> recycled. However, it turns out that when the test program enters
> main(), its VM space has some similarities with swiss cheese :-). The
> exact layout probably depends on the libc version, but in my
> particular case there are some one-page holes.
Ah, that makes sense. I wonder if that indicates memory leaks in
libc, or just random one-page internal allocations that are still in
use. I've seen the same effect on Linux by opening and closing stdio
FILEs (the buffer is mmaped for some odd reason).
> It's these one-page holes that are returned by anonmap() in
> test_3(), hence the nonconsecutive pages. When I doubled the unit
> of allocation in the test-program, the tests succeeded. But, at
> least in principle, this is just as fragile; there is no reason why
> there cannot be a couple of two-page holes.
> I don't see an easy way out. In theory test_3() is fragile, but on
> most common systems out there it seems to work fine. Until I come up
> with a solution, I think it's best to add a comment about the Hurd in
> the test and pretend that the Hurd doesn't support anonymous virtual
At one point I had a loop in test_3 that allocated page after page
until it got two consecutive ones or hit some threshold at which it
gave up. How many holes were there?