Re: Use PROT_NONE for unused virtual stack memory

[Jeroen Demeyer <jdemeyer@cage.ugent.be>]

On 2016-10-03 20:21, Bill Allombert wrote:
> > No. As far as I know, this is undocumented.
>
> This is problematic... I wrote this code following the POSIX standard.
> The code is not meant to be linux specific.

As far as I know, POSIX does not define things like overcommitting 
memory, so it couldn't be in the POSIX standard. But as you say, the new 
code is still POSIX-compliant.

> Did you try madvise(, MADV_DONTNEED) ?

Yes and it did not help. I also tried it in combination with 
mprotect(..., PROT_NONE) and it still did not behave the same as 
remapping with PROT_NONE.

> But your patch remove MAP_NORESERVE. Why ?

Because it is no longer needed: with my patch, all memory on the PARI 
stack is either really used (the actual PARI stack) or not committed 
(the virtual stack between vbot and bot, mapped with PROT_NONE). In both 
of these cases, the MAP_NORESERVE flag makes no difference.


Jeroen.