mmu_gather: move tlb flush for VM_PFNMAP/VM_MIXEDMAP vmas into free_pgtables()

Commit b67fbebd4cf9 ("mmu_gather: Force tlb-flush VM_PFNMAP vmas") added a
forced tlbflush to tlb_vma_end(), which is required to avoid a race
between munmap() and unmap_mapping_range().  However it added some
overhead to other paths where tlb_vma_end() is used, but vmas are not
removed, e.g.  madvise(MADV_DONTNEED).

Fix this by moving the tlb flush out of tlb_end_vma() into new
tlb_flush_vmas() called from free_pgtables(), somewhat similar to the
stable version of the original commit: commit 895428ee124a ("mm: Force TLB
flush for PFNMAP mappings before unlink_file_vma()").

Note, that if tlb->fullmm is set, no flush is required, as the whole mm is
about to be destroyed.

Link: https://lkml.kernel.org/r/20250522012838.163876-1-roman.gushchin@linux.dev
Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Reviewed-by: Jann Horn <jannh@google.com>
Acked-by: Hugh Dickins <hughd@google.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>
Cc: Nick Piggin <npiggin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index 88a4297..1fff717 100644 (file)
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -58,6 +58,11 @@
  *    Defaults to flushing at tlb_end_vma() to reset the range; helps when
  *    there's large holes between the VMAs.
  *
+ *  - tlb_free_vmas()
+ *
+ *    tlb_free_vmas() marks the start of unlinking of one or more vmas
+ *    and freeing page-tables.
+ *
  *  - tlb_remove_table()
  *
  *    tlb_remove_table() is the basic primitive to free page-table directories
@@ -464,7 +469,12 @@ 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));
+
+       /*
+        * Track if there's at least one VM_PFNMAP/VM_MIXEDMAP vma
+        * in the tracked range, see tlb_free_vmas().
+        */
+       tlb->vma_pfn |= !!(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP));
 }
 
 static inline void tlb_flush_mmu_tlbonly(struct mmu_gather *tlb)
@@ -547,23 +557,39 @@ 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))
+               return;
+
+       /*
+        * 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.
+        */
+       tlb_flush_mmu_tlbonly(tlb);
+}
+
+static inline void tlb_free_vmas(struct mmu_gather *tlb)
 {
        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.
+        * page mapcount -- there might not be page-frames for these PFNs
+        * after all.
+        *
+        * Specifically() there is a race between munmap() and
+        * unmap_mapping_range(), where munmap() will unlink the VMA, such
+        * that unmap_mapping_range() will no longer observe the VMA and
+        * no-op, without observing the TLBI, returning prematurely.
+        *
+        * So if we're about to unlink such a VMA, and we have pending
+        * TLBI for such a vma, flush things now.
         */
-       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.
-                */
+       if (tlb->vma_pfn)
                tlb_flush_mmu_tlbonly(tlb);
-       }
 }
 
 /*

diff --git a/mm/memory.c b/mm/memory.c
index 37d8738..8eba595 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -358,6 +358,8 @@ void free_pgtables(struct mmu_gather *tlb, struct ma_state *mas,
 {
        struct unlink_vma_file_batch vb;
 
+       tlb_free_vmas(tlb);
+
        do {
                unsigned long addr = vma->vm_start;
                struct vm_area_struct *next;

diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
index db7ba4a..b49cc63 100644 (file)
--- a/mm/mmu_gather.c
+++ b/mm/mmu_gather.c
@@ -424,6 +424,7 @@ static void __tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm,
 #ifdef CONFIG_MMU_GATHER_PAGE_SIZE
        tlb->page_size = 0;
 #endif
+       tlb->vma_pfn = 0;
 
        __tlb_reset_range(tlb);
        inc_tlb_flush_pending(tlb->mm);