[RFC patch 4/7] thp, s390: thp splitting backend for System z

Gerald Schaefer
2012-08-23T17:21:15Z

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This patch is part of the architecture backend for thp on System z.
It provides the functions related to thp splitting, including
serialization against gup. Unlike other archs, pmdp_splitting_flush()
cannot use a tlb flushing operation to serialize against gup on s390,
because that wouldn't be stopped by the disabled IRQs. So instead,
smp_call_function() is called with an empty function, which will have
the expected effect.

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
---
arch/s390/include/asm/pgtable.h | 13 +++++++++++++
arch/s390/mm/gup.c | 11 ++++++++++-
arch/s390/mm/pgtable.c | 18 ++++++++++++++++++
3 files changed, 41 insertions(+), 1 deletion(-)

--- a/arch/s390/include/asm/pgtable.h
+++ b/arch/s390/include/asm/pgtable.h
@@ -347,6 +347,8 @@ extern struct page *vmemmap;

#define _SEGMENT_ENTRY_LARGE 0x400 /* STE-format control, large page */
#define _SEGMENT_ENTRY_CO 0x100 /* change-recording override */
+#define _SEGMENT_ENTRY_SPLIT_BIT 0 /* THP splitting bit number */
+#define _SEGMENT_ENTRY_SPLIT (1UL << _SEGMENT_ENTRY_SPLIT_BIT)

/* Page status table bits for virtualization */
#define RCP_ACC_BITS 0xf000000000000000UL
@@ -506,6 +508,10 @@ static inline int pmd_bad(pmd_t pmd)
return (pmd_val(pmd) & mask) != _SEGMENT_ENTRY;
}

+#define __HAVE_ARCH_PMDP_SPLITTING_FLUSH
+extern void pmdp_splitting_flush(struct vm_area_struct *vma,
+ unsigned long addr, pmd_t *pmdp);
+
static inline int pte_none(pte_t pte)
{
return (pte_val(pte) & _PAGE_INVALID) && !(pte_val(pte) & _PAGE_SWT);
@@ -1159,6 +1165,13 @@ static inline pmd_t *pmd_offset(pud_t *p
#define pte_offset_map(pmd, address) pte_offset_kernel(pmd, address)
#define pte_unmap(pte) do { } while (0)

+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+static inline int pmd_trans_splitting(pmd_t pmd)
+{
+ return pmd_val(pmd) & _SEGMENT_ENTRY_SPLIT;
+}
+#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
+
/*
* 31 bit swap entry format:
* A page-table entry has some bits we have to treat in a special way.
--- a/arch/s390/mm/gup.c
+++ b/arch/s390/mm/gup.c
@@ -115,7 +115,16 @@ static inline int gup_pmd_range(pud_t *p
pmd = *pmdp;
barrier();
next = pmd_addr_end(addr, end);
- if (pmd_none(pmd))
+ /*
+ * The pmd_trans_splitting() check below explains why
+ * pmdp_splitting_flush() has to serialize with
+ * smp_call_function() against our disabled IRQs, to stop
+ * this gup-fast code from running while we set the
+ * splitting bit in the pmd. Returning zero will take
+ * the slow path that will call wait_split_huge_page()
+ * if the pmd is still in splitting state.
+ */
+ if (pmd_none(pmd) || pmd_trans_splitting(pmd))
return 0;
if (unlikely(pmd_huge(pmd))) {
if (!gup_huge_pmd(pmdp, pmd, addr, next,
--- a/arch/s390/mm/pgtable.c
+++ b/arch/s390/mm/pgtable.c
@@ -866,3 +866,21 @@ bool kernel_page_present(struct page *pa
return cc == 0;
}
#endif /* CONFIG_HIBERNATION && CONFIG_DEBUG_PAGEALLOC */
+
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+static void pmdp_splitting_flush_sync(void *arg)
+{
+ /* Simply deliver the interrupt */
+}
+
+void pmdp_splitting_flush(struct vm_area_struct *vma, unsigned long address,
+ pmd_t *pmdp)
+{
+ VM_BUG_ON(address & ~HPAGE_PMD_MASK);
+ if (!test_and_set_bit(_SEGMENT_ENTRY_SPLIT_BIT,
+ (unsigned long *) pmdp)) {
+ /* need to serialize against gup-fast (IRQ disabled) */
+ smp_call_function(pmdp_splitting_flush_sync, NULL, 1);
+ }
+}
+#endif /* CONFIG_TRANSPARENT_HUGEPAGE */