Commit cfe79c00 authored Jan 06, 2010 by Mike Frysinger Committed by Linus Torvalds Jan 06, 2010

NOMMU: Avoiding duplicate icache flushes of shared maps

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>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 04e4f2b1

include/linux/mm_types.h

+2 −0

Original line number	Original line	Diff line number	Diff line
	@@ -123,6 +123,8 @@ struct vm_region {
	struct file vm_file; / the backing file or NULL */		struct file vm_file; / the backing file or NULL */

	atomic_t vm_usage; /* region usage count */		atomic_t vm_usage; /* region usage count */
			bool vm_icache_flushed : 1; /* true if the icache has been flushed for
			* this region */
	};		};

	/*		/*

mm/nommu.c

+8 −3

Original line number	Original line	Diff line number	Diff line
	@@ -432,6 +432,7 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
	/*		/*
	* Ok, looks good - let it rip.		* Ok, looks good - let it rip.
	*/		*/
			flush_icache_range(mm->brk, brk);
	return mm->brk = brk;		return mm->brk = brk;
	}		}

	@@ -1353,10 +1354,14 @@ unsigned long do_mmap_pgoff(struct file *file,
	share:		share:
	add_vma_to_mm(current->mm, vma);		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)		up_write(&nommu_region_sem);
	flush_icache_range(result, result + len);

	kleave(" = %lx", result);		kleave(" = %lx", result);
	return result;		return result;

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