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Subject[patch] mm: reduce pagetable-freeing latencies

the patch below has been tested in -rt for a long time - lets get it
upstream please. It fixes some bad latencies on PREEMPT too.

Ingo

----------------------->
From: Hugh Dickins <hugh@veritas.com>
Subject: mm: reduce pagetable-freeing latencies
2.6.15-rc1 moved the unlinking of a vma from its prio_tree and anon_vma
into free_pgtables: so the vma is hidden from rmap and vmtruncate before
freeing its page tables, allowing safe descent without page table lock.
But free_pgtables is still called with preemption disabled, and Lee
Revell has now detected high latency there.

The right fix will be to rework the mmu_gathering, not to need preemption
disabled; but for now an ugly CONFIG_PREEMPT block in free_pgtables, to
make an initial unlinking pass with preemption enabled - made uglier by
CONFIG_IA64 definitions (only ia64 actually uses the start and end given
to tlb_finish_mmu, and our floor and ceiling don't quite work for those).
These CONFIG choices being to minimize the additional TLB flushing.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
--

mm/memory.c | 32 ++++++++++++++++++++++++++++++++
1 file changed, 32 insertions(+)
Index: linux-rt-rebase.q/mm/memory.c
===================================================================
--- linux-rt-rebase.q.orig/mm/memory.c
+++ linux-rt-rebase.q/mm/memory.c
@@ -264,18 +264,48 @@ void free_pgd_range(struct mmu_gather **
flush_tlb_pgtables((*tlb)->mm, start, end);
}

+#ifdef CONFIG_IA64
+#define tlb_start_addr(tlb) (tlb)->start_addr
+#define tlb_end_addr(tlb) (tlb)->end_addr
+#else
+#define tlb_start_addr(tlb) 0UL /* only ia64 really uses it */
+#define tlb_end_addr(tlb) 0UL /* only ia64 really uses it */
+#endif
+
void free_pgtables(struct mmu_gather **tlb, struct vm_area_struct *vma,
unsigned long floor, unsigned long ceiling)
{
+#ifdef CONFIG_PREEMPT
+ struct vm_area_struct *unlink = vma;
+ int fullmm = (*tlb)->fullmm;
+
+ if (!vma) /* Sometimes when exiting after an oops */
+ return;
+ if (vma->vm_next)
+ tlb_finish_mmu(*tlb, tlb_start_addr(*tlb), tlb_end_addr(*tlb));
+ /*
+ * Hide vma from rmap and vmtruncate before freeeing pgtables,
+ * with preemption enabled, except when unmapping just one area.
+ */
+ while (unlink) {
+ anon_vma_unlink(unlink);
+ unlink_file_vma(unlink);
+ unlink = unlink->vm_next;
+ }
+ if (vma->vm_next)
+ *tlb = tlb_gather_mmu(vma->vm_mm, fullmm);
+#endif
while (vma) {
struct vm_area_struct *next = vma->vm_next;
unsigned long addr = vma->vm_start;

+#ifndef CONFIG_PREEMPT
/*
* Hide vma from rmap and vmtruncate before freeing pgtables
*/
anon_vma_unlink(vma);
unlink_file_vma(vma);
+#endif

if (is_vm_hugetlb_page(vma)) {
hugetlb_free_pgd_range(tlb, addr, vma->vm_end,
@@ -288,8 +318,10 @@ void free_pgtables(struct mmu_gather **t
&& !is_vm_hugetlb_page(next)) {
vma = next;
next = vma->vm_next;
+#ifndef CONFIG_PREEMPT
anon_vma_unlink(vma);
unlink_file_vma(vma);
+#endif
}
free_pgd_range(tlb, addr, vma->vm_end,
floor, next? next->vm_start: ceiling);
-
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