On 10/25/2012 04:17 PM, Linus Torvalds wrote:

On Thu, Oct 25, 2012 at 5:16 AM, Peter Zijlstra [off-list ref] wrote:

quoted

From: Rik van Riel <redacted>

@@ -306,11 +306,26 @@ int ptep_set_access_flags(struct vm_area
                           pte_t entry, int dirty)
  {
         int changed = !pte_same(*ptep, entry);
+       /*
+        * If the page used to be inaccessible (_PAGE_PROTNONE), or
+        * this call upgrades the access permissions on the same page,
+        * it is safe to skip the remote TLB flush.
+        */
+       bool flush_remote = false;
+       if (!pte_accessible(*ptep))
+               flush_remote = false;
+       else if (pte_pfn(*ptep) != pte_pfn(entry) ||
+                       (pte_write(*ptep) && !pte_write(entry)) ||
+                       (pte_exec(*ptep) && !pte_exec(entry)))
+               flush_remote = true;

         if (changed && dirty) {

Did anybody ever actually look at this sh*t-for-brains patch?

Yeah, I'm grumpy. But I'm wasting time looking at patches that have
new code in them that is stupid and retarded.

This is the VM, guys, we don't add stupid and retarded code.

LOOK at the code, for chrissake. Just look at it. And if you don't see
why the above is stupid and retarded, you damn well shouldn't be
touching VM code.

I agree it is pretty ugly.  However, the above patch
did get rid of a gigantic performance regression with
Peter's code.

Doing unnecessary remote TLB flushes was costing about
90% performance with specjbb on a 4 node system.

However, if we can guarantee that ptep_set_access_flags
is only ever called for pte permission _upgrades_, we
can simply get rid of the remote TLB flush on x86, and
skip the paranoia tests we are doing above.

Do we have that kind of guarantee?

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