[PATCH 3/4] mm: Defer flush of writable TLB entries

Mel Gorman
2015-06-09T17:32:52Z

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>
---
include/linux/sched.h | 7 +++++++
mm/internal.h | 4 ++++
mm/rmap.c | 28 +++++++++++++++++++++-------
mm/vmscan.c | 7 ++++++-
4 files changed, 38 insertions(+), 8 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index d891e01f0445..6b787a7f6c38 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1299,6 +1299,13 @@ struct tlbflush_unmap_batch {

/* True if any bit in cpumask is set */
bool flush_required;
+
+ /*
+ * If true then the PTE was dirty when unmapped. The entry must be
+ * flushed before IO is initiated or a stale TLB entry potentially
+ * allows an update without redirtying the page.
+ */
+ bool writable;
};

struct task_struct {
diff --git a/mm/internal.h b/mm/internal.h
index 465e621b86b1..4bbe774497b2 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -438,10 +438,14 @@ struct tlbflush_unmap_batch;

#ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
void try_to_unmap_flush(void);
+void try_to_unmap_flush_dirty(void);
#else
static inline void try_to_unmap_flush(void)
{
}
+static inline void try_to_unmap_flush_dirty(void)
+{
+}

#endif /* CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH */
#endif /* __MM_INTERNAL_H */
diff --git a/mm/rmap.c b/mm/rmap.c
index 4cadb60df74a..1e36b2fb3e95 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -623,16 +623,34 @@ void try_to_unmap_flush(void)
}
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 && 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 any IO is initiated.
+ */
+ if (writable)
+ tlb_ubc->writable = true;
}

/*
@@ -655,7 +673,7 @@ static bool should_defer_flush(struct mm_struct *mm, enum ttu_flags flags)
}
#else
static void set_tlb_ubc_flush_pending(struct mm_struct *mm,
- struct page *page)
+ struct page *page, bool writable)
{
}

@@ -1307,11 +1325,7 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
*/
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);
}
diff --git a/mm/vmscan.c b/mm/vmscan.c
index f16e07aaef59..c329eca98edf 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1065,7 +1065,12 @@ static unsigned long shrink_page_list(struct list_head *page_list,
if (!sc->may_writepage)
goto keep_locked;

- /* Page is dirty, try to write it out here */
+ /*
+ * Page is dirty. Flush the TLB if a writable entry
+ * potentially exists to avoid CPU writes after IO
+ * starts and then write it out here
+ */
+ try_to_unmap_flush_dirty();
switch (pageout(page, mapping, sc)) {
case PAGE_KEEP:
goto keep_locked;
--
2.3.5