Re: [PATCH 1/3] Fix COW D-cache aliasing on fork

[Linus Torvalds]

On Fri, 20 Oct 2006, David Miller wrote:
> 
> I did some more digging, here's what I think the hardware actually
> does:

Ok, this sounds sane.

What should we do about this? How does this patch look to people?

(Totally untested, and I'm not sure we should even do that whole 
"oldmm->mm_users" test, but I'm throwing it out here for discussion, in 
case it matters for performance. The second D$ flush should obviously be 
unnecessary for the common unthreaded case, which is why none of this has 
mattered historically, I think).

Comments? We need ARM, MIPS, sparc and S390 at the very least to sign off 
on this, and somebody to write a nice explanation for the changelog (and 
preferably do this through -mm too).

		Linus

---
diff --git a/kernel/fork.c b/kernel/fork.c
index 29ebb30..14c6a1d 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -287,8 +287,18 @@ static inline int dup_mmap(struct mm_str
 	}
 	retval = 0;
 out:
-	up_write(&mm->mmap_sem);
 	flush_tlb_mm(oldmm);
+	/*
+	 * If we have other threads using the old mm, we need to
+	 * flush the D$ again - the other threads might have dirtied
+	 * it more before the TLB got flushed.
+	 *
+	 * After the flush, they can no longer dirty more pages,
+	 * since they are now marked read-only, of course.
+	 */
+	if (atomic_read(&oldmm->mm_users) != 1)
+		flush_cache_mm(oldmm);
+	up_write(&mm->mmap_sem);
 	up_write(&oldmm->mmap_sem);
 	return retval;
 fail_nomem_policy:
-
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