On Fri, May 22, 2015 at 01:18:22PM -0700, Andrew Morton wrote:

On Thu, 14 May 2015 19:31:19 +0200 Andrea Arcangeli [off-list ref] wrote:

quoted

If the rwsem starves writers it wasn't strictly a bug but lockdep
doesn't like it and this avoids depending on lowlevel implementation
details of the lock.

...

@@ -229,13 +246,33 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
 
 		if (!zeropage)
 			err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,
-					       dst_addr, src_addr);
+					       dst_addr, src_addr, &page);
 		else
 			err = mfill_zeropage_pte(dst_mm, dst_pmd, dst_vma,
 						 dst_addr);
 
 		cond_resched();
 
+		if (unlikely(err == -EFAULT)) {
+			void *page_kaddr;
+
+			BUILD_BUG_ON(zeropage);

I'm not sure what this is trying to do.  BUILD_BUG_ON(local_variable)?

It goes bang in my build.  I'll just delete it.

Yes, it has to be a false positive failure, so it's fine to drop
it. My gcc 4.8.4 can go inside the static called function and see that
only mcopy_atomic_pte can return -EFAULT. RHEL7 (4.8.3) gcc didn't
complain either. Perhaps to make the BUILD_BUG_ON work with older gcc,
it requrires a local variable set explicitly in the callee, but it's
not worth it.

It would be bad if we end up in the -EFAULT path in the zeropage case
(if somebody later adds an apparently innocent -EFAULT retval and
unexpectedly ends up in the mcopy_atomic_pte retry logic), but it's
not important, the caller should be reviewed before improvising new
retvals anyway.

The retry loop addition and the BUILD_BUG_ON is all about the
copy_from_user run while we already hold the mmap_sem (potentially of
a different process in the non-cooperative case but it's a problem if
it's the current task mmap_sem in case the rwlock implementation
changes to avoid write starvation and becomes non-reentrant). lockdep
definitely complains (even if I think in practice it'd be safe to
read-lock recurse, we just got lockdep complains never deadlocks in
fact). I didn't want to call gup_fast as copy_from_user is faster and
I got an usable user mapping with likely TLB entry hot too. The
lockdep warnings we hit I think were associated with NUMA hinting
faults or something infrequent like that, the fast path doesn't need
to retry.

Thanks,
Andrea

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