x86/mm: Eliminate redundant page table walk during TLB range flushing

When choosing between doing an address space or ranged flush,
the x86 implementation of flush_tlb_mm_range takes into account
whether there are any large pages in the range.  A per-page
flush typically requires fewer entries than would covered by a
single large page and the check is redundant.

There is one potential exception.  THP migration flushes single
THP entries and it conceivably would benefit from flushing a
single entry instead of the mm.  However, this flush is after a
THP allocation, copy and page table update potentially with any
other threads serialised behind it.  In comparison to that, the
flush is noise.  It makes more sense to optimise balancing to
require fewer flushes than to optimise the flush itself.

This patch deletes the redundant huge page check.
Signed-off-by: Mel Gorman <mgorman@suse.de>
Tested-by: Davidlohr Bueso <davidlohr@hp.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Alex Shi <alex.shi@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-sgei1drpOcburujPsfh6ovmo@git.kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/mm/tlb.c

@@ -158,32 +158,6 @@ void flush_tlb_current_task(void)
 	preempt_enable();
 }
 
-/*
- * It can find out the THP large page, or
- * HUGETLB page in tlb_flush when THP disabled
- */
-static inline unsigned long has_large_page(struct mm_struct *mm,
-				 unsigned long start, unsigned long end)
-{
-	pgd_t *pgd;
-	pud_t *pud;
-	pmd_t *pmd;
-	unsigned long addr = ALIGN(start, HPAGE_SIZE);
-	for (; addr < end; addr += HPAGE_SIZE) {
-		pgd = pgd_offset(mm, addr);
-		if (likely(!pgd_none(*pgd))) {
-			pud = pud_offset(pgd, addr);
-			if (likely(!pud_none(*pud))) {
-				pmd = pmd_offset(pud, addr);
-				if (likely(!pmd_none(*pmd)))
-					if (pmd_large(*pmd))
-						return addr;
-			}
-		}
-	}
-	return 0;
-}
-
 void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
 				unsigned long end, unsigned long vmflag)
 {
@@ -218,7 +192,7 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
 	nr_base_pages = (end - start) >> PAGE_SHIFT;
 
 	/* tlb_flushall_shift is on balance point, details in commit log */
-	if (nr_base_pages > act_entries || has_large_page(mm, start, end)) {
+	if (nr_base_pages > act_entries) {
 		count_vm_tlb_event(NR_TLB_LOCAL_FLUSH_ALL);
 		local_flush_tlb();
 	} else {