Re: missing madvise functionality

Nick Piggin
2007-04-04T08:55:18Z

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Peter Zijlstra wrote:
> On Wed, 2007-04-04 at 12:22 +1000, Nick Piggin wrote:
>
>>Eric Dumazet wrote:
>
>
>>>I do think such workloads might benefit from a vma_cache not shared by
>>>all threads but private to each thread. A sequence could invalidate the
>>>cache(s).
>>>
>>>ie instead of a mm->mmap_cache, having a mm->sequence, and each thread
>>>having a current->mmap_cache and current->mm_sequence
>>
>>I have a patchset to do exactly this, btw.
>
>
> /me too
>
> However, I decided against pushing it because when it does happen that a
> task is not involved with a vma lookup for longer than it takes the seq
> count to wrap we have a stale pointer...
>
> We could go and walk the tasks once in a while to reset the pointer, but
> it all got a tad involved.

Well here is my core patch (against I think 2.6.16 + a set of vma cache
cleanups and abstractions). I didn't think the wrapping aspect was
terribly involved.

--
SUSE Labs, Novell Inc.

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Index: linux-2.6/include/linux/sched.h
===================================================================
--- linux-2.6.orig/include/linux/sched.h
+++ linux-2.6/include/linux/sched.h
@@ -296,6 +296,8 @@ struct mm_struct {
struct vm_area_struct *mmap; /* list of VMAs */
struct rb_root mm_rb;
struct vm_area_struct *vma_cache; /* find_vma cache */
+ unsigned long vma_sequence;
+
unsigned long (*get_unmapped_area) (struct file *filp,
unsigned long addr, unsigned long len,
unsigned long pgoff, unsigned long flags);
@@ -693,6 +695,8 @@ enum sleep_type {
SLEEP_INTERRUPTED,
};

+#define VMA_CACHE_SIZE 4
+
struct task_struct {
volatile long state; /* -1 unrunnable, 0 runnable, >0 stopped */
struct thread_info *thread_info;
@@ -734,6 +738,8 @@ struct task_struct {
struct list_head ptrace_list;

struct mm_struct *mm, *active_mm;
+ struct vm_area_struct *vma_cache[VMA_CACHE_SIZE];
+ unsigned long vma_cache_sequence;

/* task state */
struct linux_binfmt *binfmt;
Index: linux-2.6/mm/mmap.c
===================================================================
--- linux-2.6.orig/mm/mmap.c
+++ linux-2.6/mm/mmap.c
@@ -32,6 +32,40 @@

static void vma_cache_touch(struct mm_struct *mm, struct vm_area_struct *vma)
{
+ struct task_struct *curr = current;
+ if (mm == curr->mm) {
+ int i;
+ if (curr->vma_cache_sequence != mm->vma_sequence) {
+ curr->vma_cache_sequence = mm->vma_sequence;
+ curr->vma_cache[0] = vma;
+ for (i = 1; i < VMA_CACHE_SIZE; i++)
+ curr->vma_cache[i] = NULL;
+ } else {
+ int update_mm;
+
+ if (curr->vma_cache[0] == vma)
+ return;
+
+ for (i = 1; i < VMA_CACHE_SIZE; i++) {
+ if (curr->vma_cache[i] == vma)
+ break;
+ }
+ update_mm = 0;
+ if (i == VMA_CACHE_SIZE) {
+ update_mm = 1;
+ i = VMA_CACHE_SIZE-1;
+ }
+ while (i != 0) {
+ curr->vma_cache[i] = curr->vma_cache[i-1];
+ i--;
+ }
+ curr->vma_cache[0] = vma;
+
+ if (!update_mm)
+ return;
+ }
+ }
+
if (mm->vma_cache != vma) /* prevent cacheline bouncing */
mm->vma_cache = vma;
}
@@ -39,27 +73,56 @@ static void vma_cache_touch(struct mm_st
static void vma_cache_replace(struct mm_struct *mm, struct vm_area_struct *vma,
struct vm_area_struct *repl)
{
+ mm->vma_sequence++;
+ if (unlikely(mm->vma_sequence == 0)) {
+ struct task_struct *curr = current, *t;
+ t = curr;
+ rcu_read_lock();
+ do {
+ t->vma_cache_sequence = -1;
+ t = next_thread(t);
+ } while (t != curr);
+ rcu_read_unlock();
+ }
+
if (mm->vma_cache == vma)
mm->vma_cache = repl;
}

static void vma_cache_invalidate(struct mm_struct *mm, struct vm_area_struct *vma)
{
- if (mm->vma_cache == vma)
- mm->vma_cache = NULL;
+ vma_cache_replace(mm, vma, NULL);
}

static struct vm_area_struct *vma_cache_find(struct mm_struct *mm,
unsigned long addr)
{
- struct vm_area_struct *vma = mm->vma_cache;
+ struct task_struct *curr;
+ struct vm_area_struct *vma;

preempt_disable();
__inc_page_state(vma_cache_query);
- if (vma && vma->vm_end > addr && vma->vm_start <= addr)
+
+ curr = current;
+ if (mm == curr->mm && mm->vma_sequence == curr->vma_cache_sequence) {
+ int i;
+ for (i = 0; i < VMA_CACHE_SIZE; i++) {
+ vma = curr->vma_cache[i];
+ if (vma && vma->vm_end > addr && vma->vm_start <= addr){
+ __inc_page_state(vma_cache_hit);
+ goto out;
+ }
+ }
+ }
+
+ vma = mm->vma_cache;
+ if (vma && vma->vm_end > addr && vma->vm_start <= addr) {
__inc_page_state(vma_cache_hit);
- else
- vma = NULL;
+ goto out;
+ }
+
+ vma = NULL;
+out:
preempt_enable();

return vma;
@@ -1439,9 +1502,9 @@ struct vm_area_struct * find_vma(struct
} else
rb_node = rb_node->rb_right;
}
- if (vma)
- vma_cache_touch(mm, vma);
}
+ if (vma)
+ vma_cache_touch(mm, vma);
}
return vma;
}
@@ -1487,6 +1550,9 @@ find_vma_prev(struct mm_struct *mm, unsi
}

out:
+ if (vma)
+ vma_cache_touch(mm, vma);
+
*pprev = prev;
return prev ? prev->vm_next : vma;
}
Index: linux-2.6/kernel/fork.c
===================================================================
--- linux-2.6.orig/kernel/fork.c
+++ linux-2.6/kernel/fork.c
@@ -198,6 +198,7 @@ static inline int dup_mmap(struct mm_str
mm->locked_vm = 0;
mm->mmap = NULL;
mm->vma_cache = NULL;
+ mm->vma_sequence = 0;
mm->free_area_cache = oldmm->mmap_base;
mm->cached_hole_size = ~0UL;
mm->map_count = 0;