[m-rev.] for review: work around bug in reusing memory zones

[Peter Wang]

On 2006-12-04, Zoltan Somogyi <zs at csse.unimelb.edu.au> wrote:
> On 01-Dec-2006, Peter Wang <wangp at students.csse.unimelb.edu.au> wrote:
> > runtime/mercury_memory_zones.c:
> > 	Add MR_dealloc_zone_memory() functions to free memory zones.
> > 
> > 	Work around the problem above by freeing memory zones when they
> > 	are not needed instead of putting them on the free list.
> > 
> > I didn't test all the variants of MR_dealloc_zone_memory.
> 
> The idea is fine. I'd rather you you chose between the old and new code
> not with "if (1)", but with #ifdef/#else/#endif.

Ok.

> > @@ -484,6 +539,9 @@ MR_construct_zone(const char *name, int 
> >      zone->MR_zone_handler = handler;
> >  #endif /* MR_CHECK_OVERFLOW_VIA_MPROTECT */
> >  
> > +    /*
> > +    ** XXX this causes a memory leak if `zone' was not newly allocated.
> > +    */
> >      zone->MR_zone_bottom = base;
> 
> I don't understand this; please expand the comment.

I have written:

/*
** XXX If `zone' is pulled off the free-list (rather than newly allocated)
** then zone->MR_zone_bottom will be pointing to a previously allocated
** memory region.  Setting it to point to `base' therefore causes a memory
** leak.  `base' should not be allocated in the first place if `zone' is
** to be reused.  See also the workaround in MR_unget_zone().
*/

Peter

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