Re: G4, 2GB RAM report (fwd)

[Bill Studenmund <wrstuden%zembu.com@localhost>]

On Wed, 9 Aug 2000, Jason R Thorpe wrote:

Oh. Ok, now I see what's going on. I'm still a little worried that we
still are calculating the number of bytes of memory in the system, which
will overflow an int and might do weird sign-extended goofyness. How about
the patch below?

For folks only on port-powerpc, the current powerpc pmap doesn't size
things well when the machine has 2GB of memory. It calculates the number
of bytes in the system in an integer quantity. 2GB > INT_MAX, so bad
things happen.

Tsubai? WS?

Take care,

Bill

>  > 1. int physmem:
>  > 
>  >    the page hash table is auto-sized in pmap_bootstrap()\pmap.c:
>  > 
>  >    #ifdef  HTABENTS
>  >            ptab_cnt = HTABENTS;
>  >    #else /* HTABENTS */
>  >            ptab_cnt = 1024;
>  >            while ((HTABSIZE << 7) < ctob(physmem))
>  >                    ptab_cnt <<= 1;
>  >    #endif /* HTABENTS */
>  > 
>  >    with >= 2GB RAM, ctob(physmem) as a signed quantity 
>  >    will evaluate negative right off so ptab_cnt never gets
>  >    adjusted upwards.  The result is overflow of the (puny)
>  >    page table during startup before VM is initialized
>  >    which results in panic("poalloc").

Index: pmap.c
===================================================================
RCS file: /cvsroot/syssrc/sys/arch/powerpc/powerpc/pmap.c,v
retrieving revision 1.30
diff -u -r1.30 pmap.c
--- pmap.c      2000/06/29 07:48:18     1.30
+++ pmap.c      2000/08/09 20:37:04
@@ -47,6 +47,12 @@
 int ptab_cnt;
 u_int ptab_mask;
 #define        HTABSIZE        (ptab_cnt * 64)
+#define        HTABTOPAGES     (ptab_cnt << (6 + 7 - PGSHIFT))
+/*
+ * above calculates btoc(HTABSIZE << 7) without overflow. Assumes PGSHIFT
+ * is not greater than 13, which means that our physical pages aren't
+ * larger than 16k. They are 8k at present.
+ */
 
 struct pte_ovfl {
        LIST_ENTRY(pte_ovfl) po_list;   /* Linked list of overflow entries */
@@ -401,7 +407,7 @@
        ptab_cnt = HTABENTS;
 #else /* HTABENTS */
        ptab_cnt = 1024;
-       while ((HTABSIZE << 7) < ctob(physmem))
+       while (HTABTOPAGES < physmem)
                ptab_cnt <<= 1;
 #endif /* HTABENTS */