mmu_gather: Force tlb-flush VM_PFNMAP vmas

Jann reported a race between munmap() and unmap_mapping_range(), where
unmap_mapping_range() will no-op once unmap_vmas() has unlinked the
VMA; however munmap() will not yet have invalidated the TLBs.

Therefore unmap_mapping_range() will complete while there are still
(stale) TLB entries for the specified range.

Mitigate this by force flushing TLBs for VM_PFNMAP ranges.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/asm-generic/tlb.h

@@ -303,6 +303,7 @@ struct mmu_gather {
 	 */
 	unsigned int		vma_exec : 1;
 	unsigned int		vma_huge : 1;
+	unsigned int		vma_pfn  : 1;
 
 	unsigned int		batch_count;
 
@@ -373,7 +374,6 @@ tlb_update_vma_flags(struct mmu_gather *tlb, struct vm_area_struct *vma) { }
 #else /* CONFIG_MMU_GATHER_NO_RANGE */
 
 #ifndef tlb_flush
-
 /*
  * When an architecture does not provide its own tlb_flush() implementation
  * but does have a reasonably efficient flush_vma_range() implementation
@@ -393,6 +393,9 @@ static inline void tlb_flush(struct mmu_gather *tlb)
 		flush_tlb_range(&vma, tlb->start, tlb->end);
 	}
 }
+#endif
+
+#endif /* CONFIG_MMU_GATHER_NO_RANGE */
 
 static inline void
 tlb_update_vma_flags(struct mmu_gather *tlb, struct vm_area_struct *vma)
@@ -410,17 +413,9 @@ tlb_update_vma_flags(struct mmu_gather *tlb, struct vm_area_struct *vma)
 	 */
 	tlb->vma_huge = is_vm_hugetlb_page(vma);
 	tlb->vma_exec = !!(vma->vm_flags & VM_EXEC);
+	tlb->vma_pfn  = !!(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP));
 }
 
-#else
-
-static inline void
-tlb_update_vma_flags(struct mmu_gather *tlb, struct vm_area_struct *vma) { }
-
-#endif
-
-#endif /* CONFIG_MMU_GATHER_NO_RANGE */
-
 static inline void tlb_flush_mmu_tlbonly(struct mmu_gather *tlb)
 {
 	/*
@@ -507,16 +502,22 @@ static inline void tlb_start_vma(struct mmu_gather *tlb, struct vm_area_struct *
 
 static inline void tlb_end_vma(struct mmu_gather *tlb, struct vm_area_struct *vma)
 {
-	if (tlb->fullmm || IS_ENABLED(CONFIG_MMU_GATHER_MERGE_VMAS))
+	if (tlb->fullmm)
 		return;
 
+	/*
+	 * VM_PFNMAP is more fragile because the core mm will not track the
+	 * page mapcount -- there might not be page-frames for these PFNs after
+	 * all. Force flush TLBs for such ranges to avoid munmap() vs
+	 * unmap_mapping_range() races.
+	 */
+	if (tlb->vma_pfn || !IS_ENABLED(CONFIG_MMU_GATHER_MERGE_VMAS)) {
 		/*
 		 * Do a TLB flush and reset the range at VMA boundaries; this avoids
-	 * the ranges growing with the unused space between consecutive VMAs,
-	 * but also the mmu_gather::vma_* flags from tlb_start_vma() rely on
-	 * this.
+		 * the ranges growing with the unused space between consecutive VMAs.
 		 */
 		tlb_flush_mmu_tlbonly(tlb);
+	}
 }
 
 /*