vmalloc: better detect default region size on Linux

This is a Linux-specific fix for vmalloc from OpenSUSE, for what
that's still worth; we're not going to keep using vmalloc long term.
https://build.opensuse.org/package/view_file/shells/ksh/ksh93-vm.dif

src/lib/libast/vmalloc/vmhdr.h

@@ -152,6 +152,9 @@ extern void		_vmmessage _ARG_((const char*, long, const char*, long));
 #endif /*DEBUG*/
 
 #define VMPAGESIZE	8192
+#if defined(__linux__) && !defined(_lib_getpagesize)
+#define _lib_getpagesize	1
+#endif
 #if _lib_getpagesize
 #define GETPAGESIZE(x)	((x) ? (x) : ((x)=getpagesize()) )
 #else

src/lib/libast/vmalloc/vmmopen.c

@@ -63,8 +63,10 @@ void _STUB_vmmapopen(){}
 /* magic word signaling file/segment is ready */
 #define	MM_MAGIC	((unsigned int)(('P'<<24) | ('&'<<16) | ('N'<<8) | ('8')) )
 
+#ifndef __linux__
 /* default minimum region size */
 #define MM_MINSIZE	(64*_Vmpagesize)
+#endif
 
 /* macros to get the data section and size */
 #define MMHEAD(file)	ROUND(sizeof(Mmvm_t)+strlen(file), ALIGN)
@@ -167,9 +169,18 @@ static int mminit(Mmdisc_t* mmdc)
 	if(mmdc->mmvm) /* already done this */
 		return 0;
 
+#ifndef __linux__
 	/* fixed size region so make it reasonably large */
 	if((size = mmdc->size) < MM_MINSIZE )
 		size =  MM_MINSIZE;
+#else /* on Linux */
+	if (sizeof(void*) > 32)
+		extent = ROUND(0x80000,_Vmpagesize);
+	else
+		extent = ROUND(0x40000,_Vmpagesize);
+	if((size = mmdc->size) < extent)
+		size = extent;
+#endif
 	size += MMHEAD(mmdc->file) + ALIGN;
 	size  = ROUND(size, _Vmpagesize);