commit d950c9477d51f0cefc2ed3cf76e695d46af0d9c1
author Mel Gorman <mgorman@suse.de> Fri Sep 04 15:47:35 2015 -0700
committer Linus Torvalds <torvalds@linux-foundation.org> Fri Sep 04 16:54:41 2015 -0700
tree d6ba89db328ec09b2140a84dcdf1bd8ca5cbeaa4
parent 72b252aed506b8f1a03f7abd29caef4cdf6a043b

mm: defer flush of writable TLB entries

If a PTE is unmapped and it's dirty then it was writable recently.  Due to
deferred TLB flushing, it's best to assume a writable TLB cache entry
exists.  With that assumption, the TLB must be flushed before any IO can
start or the page is freed to avoid lost writes or data corruption.  This
patch defers flushing of potentially writable TLBs as long as possible.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/rmap.c

diff --git a/mm/rmap.c b/mm/rmap.c
index 326d5d8..0db38e7 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -626,16 +626,34 @@
 	}
 	cpumask_clear(&tlb_ubc->cpumask);
 	tlb_ubc->flush_required = false;
+	tlb_ubc->writable = false;
 	put_cpu();
 }
 
+/* Flush iff there are potentially writable TLB entries that can race with IO */
+void try_to_unmap_flush_dirty(void)
+{
+	struct tlbflush_unmap_batch *tlb_ubc = &current->tlb_ubc;
+
+	if (tlb_ubc->writable)
+		try_to_unmap_flush();
+}
+
 static void set_tlb_ubc_flush_pending(struct mm_struct *mm,
-		struct page *page)
+		struct page *page, bool writable)
 {
 	struct tlbflush_unmap_batch *tlb_ubc = &current->tlb_ubc;
 
 	cpumask_or(&tlb_ubc->cpumask, &tlb_ubc->cpumask, mm_cpumask(mm));
 	tlb_ubc->flush_required = true;
+
+	/*
+	 * If the PTE was dirty then it's best to assume it's writable. The
+	 * caller must use try_to_unmap_flush_dirty() or try_to_unmap_flush()
+	 * before the page is queued for IO.
+	 */
+	if (writable)
+		tlb_ubc->writable = true;
 }
 
 /*
@@ -658,7 +676,7 @@
 }
 #else
 static void set_tlb_ubc_flush_pending(struct mm_struct *mm,
-		struct page *page)
+		struct page *page, bool writable)
 {
 }
 
@@ -1315,11 +1333,7 @@
 		 */
 		pteval = ptep_get_and_clear(mm, address, pte);
 
-		/* Potentially writable TLBs must be flushed before IO */
-		if (pte_dirty(pteval))
-			flush_tlb_page(vma, address);
-		else
-			set_tlb_ubc_flush_pending(mm, page);
+		set_tlb_ubc_flush_pending(mm, page, pte_dirty(pteval));
 	} else {
 		pteval = ptep_clear_flush(vma, address, pte);
 	}