x86/mm/tlb: Ignore f->new_tlb_gen when zero

Commit aa44284960 ("x86/mm/tlb: Avoid reading mm_tlb_gen when
possible") introduced an optimization to skip superfluous TLB
flushes based on the generation provided in flush_tlb_info.

However, arch_tlbbatch_flush() does not provide any generation in
flush_tlb_info and populates the flush_tlb_info generation with
0.  This 0 is causes the flush_tlb_info to be interpreted as a
superfluous, old flush.  As a result, try_to_unmap_one() would
not perform any TLB flushes.

Fix it by checking whether f->new_tlb_gen is nonzero. Zero value
is anyhow is an invalid generation value. To avoid future
confusion, introduce TLB_GENERATION_INVALID constant and use it
properly. Add warnings to ensure no partial flushes are done with
TLB_GENERATION_INVALID or when f->mm is NULL, since this does not
make any sense.

In addition, add the missing unlikely().

[ dhansen: change VM_BUG_ON() -> VM_WARN_ON(), clarify changelog ]

Fixes: aa44284960 ("x86/mm/tlb: Avoid reading mm_tlb_gen when possible")
Reported-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Nadav Amit <namit@vmware.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Tested-by: Hugh Dickins <hughd@google.com>
Link: https://lkml.kernel.org/r/20220710232837.3618-1-namit@vmware.com

arch/x86/include/asm/tlbflush.h

@@ -16,6 +16,7 @@
 void __flush_tlb_all(void);
 
 #define TLB_FLUSH_ALL	-1UL
+#define TLB_GENERATION_INVALID	0
 
 void cr4_update_irqsoff(unsigned long set, unsigned long clear);
 unsigned long cr4_read_shadow(void);

arch/x86/mm/tlb.c

@@ -771,7 +771,8 @@ static void flush_tlb_func(void *info)
 		return;
 	}
 
-	if (f->new_tlb_gen <= local_tlb_gen) {
+	if (unlikely(f->new_tlb_gen != TLB_GENERATION_INVALID &&
+		     f->new_tlb_gen <= local_tlb_gen)) {
 		/*
 		 * The TLB is already up to date in respect to f->new_tlb_gen.
 		 * While the core might be still behind mm_tlb_gen, checking
@@ -843,6 +844,12 @@ static void flush_tlb_func(void *info)
 		/* Partial flush */
 		unsigned long addr = f->start;
 
+		/* Partial flush cannot have invalid generations */
+		VM_WARN_ON(f->new_tlb_gen == TLB_GENERATION_INVALID);
+
+		/* Partial flush must have valid mm */
+		VM_WARN_ON(f->mm == NULL);
+
 		nr_invalidate = (f->end - f->start) >> f->stride_shift;
 
 		while (addr < f->end) {
@@ -1045,7 +1052,8 @@ void flush_tlb_kernel_range(unsigned long start, unsigned long end)
 		struct flush_tlb_info *info;
 
 		preempt_disable();
-		info = get_flush_tlb_info(NULL, start, end, 0, false, 0);
+		info = get_flush_tlb_info(NULL, start, end, 0, false,
+					  TLB_GENERATION_INVALID);
 
 		on_each_cpu(do_kernel_range_flush, info, 1);
 
@@ -1214,7 +1222,8 @@ void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
 
 	int cpu = get_cpu();
 
-	info = get_flush_tlb_info(NULL, 0, TLB_FLUSH_ALL, 0, false, 0);
+	info = get_flush_tlb_info(NULL, 0, TLB_FLUSH_ALL, 0, false,
+				  TLB_GENERATION_INVALID);
 	/*
 	 * flush_tlb_multi() is not optimized for the common case in which only
 	 * a local TLB flush is needed. Optimize this use-case by calling