[PATCH 5/7] NOMMU: Avoiding duplicate icache flushes of shared maps

[David Howells <dhowells@redhat.com>]

From: Mike Frysinger <vapier.adi@gmail.com>

When working with FDPIC, there are many shared mappings of read-only code
regions between applications (the C library, applet packages like busybox,
etc.), but the current do_mmap_pgoff() function will issue an icache flush
whenever a VMA is added to an MM instead of only doing it when the map is
initially created.

The flush can instead be done when a region is first mmapped PROT_EXEC.  Note
that we may not rely on the first mapping of a region being executable - it's
possible for it to be PROT_READ only, so we have to remember whether we've
flushed the region or not, and then flush the entire region when a bit of it is
made executable.

However, this also affects the brk area.  That will no longer be executable.
We can mprotect() it to PROT_EXEC on MPU-mode kernels, but for NOMMU mode
kernels, when it increases the brk allocation, making sys_brk() flush the extra
from the icache should suffice.  The brk area probably isn't used by NOMMU
programs since the brk area can only use up the leavings from the stack
allocation, where the stack allocation is larger than requested.

Signed-off-by: David Howells <dhowells@redhat.com>
---

 include/linux/mm_types.h |    2 ++
 mm/nommu.c               |   11 ++++++++---
 2 files changed, 10 insertions(+), 3 deletions(-)


diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 84a524a..84d020b 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -123,6 +123,8 @@ struct vm_region {
 	struct file	*vm_file;	/* the backing file or NULL */
 
 	atomic_t	vm_usage;	/* region usage count */
+	bool		vm_icache_flushed : 1; /* true if the icache has been flushed for
+						* this region */
 };
 
 /*
diff --git a/mm/nommu.c b/mm/nommu.c
index 1544a65..bffbdfc 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -432,6 +432,7 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
 	/*
 	 * Ok, looks good - let it rip.
 	 */
+	flush_icache_range(mm->brk, brk);
 	return mm->brk = brk;
 }
 
@@ -1354,10 +1355,14 @@ unsigned long do_mmap_pgoff(struct file *file,
 share:
 	add_vma_to_mm(current->mm, vma);
 
-	up_write(&nommu_region_sem);
+	/* we flush the region from the icache only when the first executable
+	 * mapping of it is made  */
+	if (vma->vm_flags & VM_EXEC && !region->vm_icache_flushed) {
+		flush_icache_range(region->vm_start, region->vm_end);
+		region->vm_icache_flushed = true;
+	}
 
-	if (prot & PROT_EXEC)
-		flush_icache_range(result, result + len);
+	up_write(&nommu_region_sem);
 
 	kleave(" = %lx", result);
 	return result;