uml: runtime host VMSPLIT detection

Calculate TASK_SIZE at run-time by figuring out the host's VMSPLIT - this is
needed on i386 if UML is to run on hosts with varying VMSPLITs without
recompilation.

TASK_SIZE is now defined in terms of a variable, task_size.  This gets rid of
an include of pgtable.h from processor.h, which can cause include loops.

On i386, task_size is calculated early in boot by probing the address space in
a binary search to figure out where the boundary between usable and non-usable
memory is.  This tries to make sure that a page that is considered to be in
userspace is, or can be made, read-write.  I'm concerned about a system-global
VDSO page in kernel memory being hit and considered to be a userspace page.

On x86_64, task_size is just the old value of CONFIG_TOP_ADDR.

A bunch of config variable are gone now.  CONFIG_TOP_ADDR is directly replaced
by TASK_SIZE.  NEST_LEVEL is gone since the relocation of the stubs makes it
irrelevant.  All the HOST_VMSPLIT stuff is gone.  All references to these in
arch/um/Makefile are also gone.

I noticed and fixed a missing extern in os.h when adding os_get_task_size.

Note: This has been revised to fix the 32-bit UML on 64-bit host bug that
Miklos ran into.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 15 insertions, 3 deletions

diff --git a/arch/um/kernel/exec.c b/arch/um/kernel/exec.c
index 76a62c0cb2bc..f5d7f4569ba7 100644
--- a/arch/um/kernel/exec.c
+++ b/arch/um/kernel/exec.c
@@ -25,7 +25,7 @@ void flush_thread(void)
 
 	ret = unmap(&current->mm->context.id, 0, STUB_START, 0, &data);
 	ret = ret || unmap(&current->mm->context.id, STUB_END,
-			   TASK_SIZE - STUB_END, 1, &data);
+			   host_task_size - STUB_END, 1, &data);
 	if (ret) {
 		printk(KERN_ERR "flush_thread - clearing address space failed, "
 		       "err = %d\n", ret);
diff --git a/arch/um/kernel/um_arch.c b/arch/um/kernel/um_arch.c
index 468aba990dbd..a6c1dd1cf5a1 100644
--- a/arch/um/kernel/um_arch.c
+++ b/arch/um/kernel/um_arch.c
@@ -241,6 +241,11 @@ static struct notifier_block panic_exit_notifier = {
 };
 
 /* Set during early boot */
+unsigned long task_size;
+EXPORT_SYMBOL(task_size);
+
+unsigned long host_task_size;
+
 unsigned long brk_start;
 unsigned long end_iomem;
 EXPORT_SYMBOL(end_iomem);
@@ -267,6 +272,13 @@ int __init linux_main(int argc, char **argv)
 	if (have_root == 0)
 		add_arg(DEFAULT_COMMAND_LINE);
 
+	host_task_size = os_get_task_size();
+	/*
+	 * TASK_SIZE needs to be PGDIR_SIZE aligned or else exit_mmap craps
+	 * out
+	 */
+	task_size = host_task_size & PGDIR_MASK;
+
 	/* OS sanity checks that need to happen before the kernel runs */
 	os_early_checks();
 
@@ -303,7 +315,7 @@ int __init linux_main(int argc, char **argv)
 
 	highmem = 0;
 	iomem_size = (iomem_size + PAGE_SIZE - 1) & PAGE_MASK;
-	max_physmem = CONFIG_TOP_ADDR - uml_physmem - iomem_size - MIN_VMALLOC;
+	max_physmem = TASK_SIZE - uml_physmem - iomem_size - MIN_VMALLOC;
 
 	/*
 	 * Zones have to begin on a 1 << MAX_ORDER page boundary,
@@ -335,7 +347,7 @@ int __init linux_main(int argc, char **argv)
 	}
 
 	virtmem_size = physmem_size;
-	avail = CONFIG_TOP_ADDR - start_vm;
+	avail = TASK_SIZE - start_vm;
 	if (physmem_size > avail)
 		virtmem_size = avail;
 	end_vm = start_vm + virtmem_size;