Re: [patch 2/2]: introduce fast_gup

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2008-04-21 at 16:26 +0300, Avi Kivity wrote:
> Peter Zijlstra wrote:
> > On Mon, 2008-04-21 at 15:00 +0300, Avi Kivity wrote:
> >   
> >> Linus Torvalds wrote:
> >>     
> >>> Finally, I don't think that comment is correct in the first place. It's 
> >>> not that simple. The thing is, even *with* the memory barrier in place, we 
> >>> may have:
> >>>
> >>> 	CPU#1			CPU#2
> >>> 	=====			=====
> >>>
> >>> 	fast_gup:
> >>> 	 - read low word
> >>>
> >>> 				native_set_pte_present:
> >>> 				 - set low word to 0
> >>> 				 - set high word to new value
> >>>
> >>> 	 - read high word
> >>>
> >>> 				- set low word to new value
> >>>
> >>> and so you read a low word that is associated with a *different* high 
> >>> word! Notice?
> >>>
> >>> So trivial memory ordering is _not_ enough.
> >>>
> >>> So I think the code literally needs to be something like this
> >>>
> >>> 	#ifdef CONFIG_X86_PAE
> >>>
> >>> 	static inline pte_t native_get_pte(pte_t *ptep)
> >>> 	{
> >>> 		pte_t pte;
> >>>
> >>> 	retry:
> >>> 		pte.pte_low = ptep->pte_low;
> >>> 		smp_rmb();
> >>> 		pte.pte_high = ptep->pte_high;
> >>> 		smp_rmb();
> >>> 		if (unlikely(pte.pte_low != ptep->pte_low)
> >>> 			goto retry;
> >>> 		return pte;
> >>> 	}
> >>>
> >>>   
> >>>       
> >> I think this is still broken.  Suppose that after reading pte_high 
> >> native_set_pte() is called again on another cpu, changing pte_low back 
> >> to the original value (but with a different pte_high).  You now have 
> >> pte_low from second native_set_pte() but pte_high from the first 
> >> native_set_pte().
> >>     
> >
> > I think the idea was that for user pages we only use set_pte_present()
> > which does the low=0 thing first.
> >   
> 
> Doesn't matter.  The second native_set_pte() (or set_pte_present()) 
> executes atomically:
> 
> 
> 	fast_gup:
> 	 - read low word (l0)
> 
> 				native_set_pte_present:
> 				 - set low word to 0
> 				 - set high word to new value (h1)
> 	 			 - set low word to new value (l1)
>  
> 
> 	 - read high word (h1)
> 
> 				native_set_pte_present:
> 				 - set low word to 0
> 				 - set high word to new value (h2)
> 	 			 - set low word to new value (l2)
> 
>    	 - re-read low word (l2)
> 
> 
> If l2 happens to be equal to l0, then the check succeeds and we have a 
> splintered pte h1:l0.

ok, so lets use cmpxchg8.

Index: linux-2.6/arch/x86/mm/gup.c
===================================================================
--- linux-2.6.orig/arch/x86/mm/gup.c
+++ linux-2.6/arch/x86/mm/gup.c
@@ -9,6 +9,19 @@
 #include <linux/vmstat.h>
 #include <asm/pgtable.h>
 
+#ifdef CONFIG_X86_PAE
+
+static inline pte_t native_get_pte(pte_t *ptep)
+{
+	return native_make_pte(cmpxchg64((u64 *)ptep, 0ULL, 0ULL));
+}
+
+#else
+
+#define native_get_pte(ptep) (*(ptep))
+
+#endif
+
 /*
  * The performance critical leaf functions are made noinline otherwise gcc
  * inlines everything into a single function which results in too much
@@ -27,16 +40,7 @@ static noinline int gup_pte_range(pmd_t 
 
 	ptep = pte_offset_map(&pmd, addr);
 	do {
-		/*
-		 * XXX: careful. On 3-level 32-bit, the pte is 64 bits, and
-		 * we need to make sure we load the low word first, then the
-		 * high. This means _PAGE_PRESENT should be clear if the high
-		 * word was not valid. Currently, the C compiler can issue
-		 * the loads in any order, and I don't know of a wrapper
-		 * function that will do this properly, so it is broken on
-		 * 32-bit 3-level for the moment.
-		 */
-		pte_t pte = *ptep;
+		pte_t pte = native_get_pte(ptep);
 		struct page *page;
 
 		if ((pte_val(pte) & mask) != result)


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