Re: Should use of mallopt depend on DOUG_LEA_MALLOC?

[Paul Eggert]

On 12/13/19 7:22 AM, Stefan Monnier wrote:
> For those in `init_alloc_once_for_pdumper`, I can't help you.

Those mallopt calls shouldn't be needed.

> But the ones conditionalized on `mmap_lisp_allowed_p` are used not as
> optimizations but to prevent use of mmap within the malloc
> implementation in the two known cases where it can break our
> (unportable) assumptions:

Yes, we need to keep those only for the vanishingly small cases where we 
use unexec and a Doug Lea-like malloc.

I looked through Emacs's uses of mallopt. There's a lot of cargo-cult 
cruft there that should go. Plus, I expect there's a real bug with newer 
glibc (so mallopt isn't called) but unexec is still used (because the 
builder asked for it - a mistake in my view) and where Emacs doesn't 
disable mmap properly.

Proposed patch attached. The basic idea is to use "#ifdef M_MMAP_MAX" 
instead of "#ifdef DOUG_LEA_MALLOC" to decide whether to call mallopt 
with M_MMAP_MAX. Plus, stop fiddling with mallopt parameters that are 
problematic given that glibc malloc has mutated so much (and is likely 
to mutate further).

0001-Simplify-use-of-mallopt.patch
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