[PATCH 5.15 267/279] hugetlbfs: flush TLBs correctly after huge_pmd_unshare

Greg Kroah-Hartman
2021-11-24T14:07:39Z

From: Nadav Amit <namit@vmware.com>

commit a4a118f2eead1d6c49e00765de89878288d4b890 upstream.

When __unmap_hugepage_range() calls to huge_pmd_unshare() succeed, a TLB
flush is missing. This TLB flush must be performed before releasing the
i_mmap_rwsem, in order to prevent an unshared PMDs page from being
released and reused before the TLB flush took place.

Arguably, a comprehensive solution would use mmu_gather interface to
batch the TLB flushes and the PMDs page release, however it is not an
easy solution: (1) try_to_unmap_one() and try_to_migrate_one() also call
huge_pmd_unshare() and they cannot use the mmu_gather interface; and (2)
deferring the release of the page reference for the PMDs page until
after i_mmap_rwsem is dropeed can confuse huge_pmd_unshare() into
thinking PMDs are shared when they are not.

Fix __unmap_hugepage_range() by adding the missing TLB flush, and
forcing a flush when unshare is successful.

Fixes: 24669e58477e ("hugetlb: use mmu_gather instead of a temporary linked list for accumulating pages)" # 3.6
Signed-off-by: Nadav Amit <namit@vmware.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
mm/hugetlb.c | 23 +++++++++++++++++++----
1 file changed, 19 insertions(+), 4 deletions(-)

--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4439,6 +4439,7 @@ void __unmap_hugepage_range(struct mmu_g
struct hstate *h = hstate_vma(vma);
unsigned long sz = huge_page_size(h);
struct mmu_notifier_range range;
+ bool force_flush = false;

WARN_ON(!is_vm_hugetlb_page(vma));
BUG_ON(start & ~huge_page_mask(h));
@@ -4467,10 +4468,8 @@ void __unmap_hugepage_range(struct mmu_g
ptl = huge_pte_lock(h, mm, ptep);
if (huge_pmd_unshare(mm, vma, &address, ptep)) {
spin_unlock(ptl);
- /*
- * We just unmapped a page of PMDs by clearing a PUD.
- * The caller's TLB flush range should cover this area.
- */
+ tlb_flush_pmd_range(tlb, address & PUD_MASK, PUD_SIZE);
+ force_flush = true;
continue;
}

@@ -4527,6 +4526,22 @@ void __unmap_hugepage_range(struct mmu_g
}
mmu_notifier_invalidate_range_end(&range);
tlb_end_vma(tlb, vma);
+
+ /*
+ * If we unshared PMDs, the TLB flush was not recorded in mmu_gather. We
+ * could defer the flush until now, since by holding i_mmap_rwsem we
+ * guaranteed that the last refernece would not be dropped. But we must
+ * do the flushing before we return, as otherwise i_mmap_rwsem will be
+ * dropped and the last reference to the shared PMDs page might be
+ * dropped as well.
+ *
+ * In theory we could defer the freeing of the PMD pages as well, but
+ * huge_pmd_unshare() relies on the exact page_count for the PMD page to
+ * detect sharing, so we cannot defer the release of the page either.
+ * Instead, do flush now.
+ */
+ if (force_flush)
+ tlb_flush_mmu_tlbonly(tlb);
}

void __unmap_hugepage_range_final(struct mmu_gather *tlb,