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Subject[PATCH v6 3/7] Revert "mm: numa: defer TLB flush for THP migration as long as possible"
Date
While deferring TLB flushes is a good practice, the reverted patch
caused pending TLB flushes to be checked while the page-table lock is
not taken. As a result, in architectures with weak memory model (PPC),
Linux may miss a memory-barrier, miss the fact TLB flushes are pending,
and cause (in theory) a memory corruption.

Since the alternative of using smp_mb__after_unlock_lock() was
considered a bit open-coded, and the performance impact is expected to
be small, the previous patch is reverted.

This reverts commit b0943d61b8fa420180f92f64ef67662b4f6cc493.

Suggested-by: Mel Gorman <mgorman@suse.de>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Nadav Amit <namit@vmware.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Acked-by: Rik van Riel <riel@redhat.com>
---
mm/huge_memory.c | 7 +++++++
mm/migrate.c | 6 ------
2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 88c6167f194d..b51d83e410eb 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1496,6 +1496,13 @@ int do_huge_pmd_numa_page(struct vm_fault *vmf, pmd_t pmd)
}

/*
+ * The page_table_lock above provides a memory barrier
+ * with change_protection_range.
+ */
+ if (mm_tlb_flush_pending(vma->vm_mm))
+ flush_tlb_range(vma, haddr, haddr + HPAGE_PMD_SIZE);
+
+ /*
* Migrate the THP to the requested node, returns with page unlocked
* and access rights restored.
*/
diff --git a/mm/migrate.c b/mm/migrate.c
index 89a0a1707f4c..1f6c2f41b3cb 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1935,12 +1935,6 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
put_page(new_page);
goto out_fail;
}
- /*
- * We are not sure a pending tlb flush here is for a huge page
- * mapping or not. Hence use the tlb range variant
- */
- if (mm_tlb_flush_pending(mm))
- flush_tlb_range(vma, mmun_start, mmun_end);

/* Prepare a page as a migration target */
__SetPageLocked(new_page);
--
2.11.0
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