remove ZERO_PAGE

The commit b5810039a54e5babf428e9a1e89fc1940fabff11 contains the note

  A last caveat: the ZERO_PAGE is now refcounted and managed with rmap
  (and thus mapcounted and count towards shared rss).  These writes to
  the struct page could cause excessive cacheline bouncing on big
  systems.  There are a number of ways this could be addressed if it is
  an issue.

And indeed this cacheline bouncing has shown up on large SGI systems.
There was a situation where an Altix system was essentially livelocked
tearing down ZERO_PAGE pagetables when an HPC app aborted during startup.
This situation can be avoided in userspace, but it does highlight the
potential scalability problem with refcounting ZERO_PAGE, and corner
cases where it can really hurt (we don't want the system to livelock!).

There are several broad ways to fix this problem:
1. add back some special casing to avoid refcounting ZERO_PAGE
2. per-node or per-cpu ZERO_PAGES
3. remove the ZERO_PAGE completely

I will argue for 3. The others should also fix the problem, but they
result in more complex code than does 3, with little or no real benefit
that I can see.

Why? Inserting a ZERO_PAGE for anonymous read faults appears to be a
false optimisation: if an application is performance critical, it would
not be doing many read faults of new memory, or at least it could be
expected to write to that memory soon afterwards. If cache or memory use
is critical, it should not be working with a significant number of
ZERO_PAGEs anyway (a more compact representation of zeroes should be
used).

As a sanity check -- mesuring on my desktop system, there are never many
mappings to the ZERO_PAGE (eg. 2 or 3), thus memory usage here should not
increase much without it.

When running a make -j4 kernel compile on my dual core system, there are
about 1,000 mappings to the ZERO_PAGE created per second, but about 1,000
ZERO_PAGE COW faults per second (less than 1 ZERO_PAGE mapping per second
is torn down without being COWed). So removing ZERO_PAGE will save 1,000
page faults per second when running kbuild, while keeping it only saves
less than 1 page clearing operation per second. 1 page clear is cheaper
than a thousand faults, presumably, so there isn't an obvious loss.

Neither the logical argument nor these basic tests give a guarantee of no
regressions. However, this is a reasonable opportunity to try to remove
the ZERO_PAGE from the pagefault path. If it is found to cause regressions,
we can reintroduce it and just avoid refcounting it.

The /dev/zero ZERO_PAGE usage and TLB tricks also get nuked.  I don't see
much use to them except on benchmarks.  All other users of ZERO_PAGE are
converted just to use ZERO_PAGE(0) for simplicity. We can look at
replacing them all and maybe ripping out ZERO_PAGE completely when we are
more satisfied with this solution.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus "snif" Torvalds <torvalds@linux-foundation.org>

1 files changed, 17 insertions, 108 deletions

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index bbee97f..64551ab 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -625,65 +625,10 @@ static ssize_t splice_write_null(struct pipe_inode_info *pipe,struct file *out,
 	return splice_from_pipe(pipe, out, ppos, len, flags, pipe_to_null);
 }
 
-#ifdef CONFIG_MMU
-/*
- * For fun, we are using the MMU for this.
- */
-static inline size_t read_zero_pagealigned(char __user * buf, size_t size)
-{
-	struct mm_struct *mm;
-	struct vm_area_struct * vma;
-	unsigned long addr=(unsigned long)buf;
-
-	mm = current->mm;
-	/* Oops, this was forgotten before. -ben */
-	down_read(&mm->mmap_sem);
-
-	/* For private mappings, just map in zero pages. */
-	for (vma = find_vma(mm, addr); vma; vma = vma->vm_next) {
-		unsigned long count;
-
-		if (vma->vm_start > addr || (vma->vm_flags & VM_WRITE) == 0)
-			goto out_up;
-		if (vma->vm_flags & (VM_SHARED | VM_HUGETLB))
-			break;
-		count = vma->vm_end - addr;
-		if (count > size)
-			count = size;
-
-		zap_page_range(vma, addr, count, NULL);
-        	if (zeromap_page_range(vma, addr, count, PAGE_COPY))
-			break;
-
-		size -= count;
-		buf += count;
-		addr += count;
-		if (size == 0)
-			goto out_up;
-	}
-
-	up_read(&mm->mmap_sem);
-	
-	/* The shared case is hard. Let's do the conventional zeroing. */ 
-	do {
-		unsigned long unwritten = clear_user(buf, PAGE_SIZE);
-		if (unwritten)
-			return size + unwritten - PAGE_SIZE;
-		cond_resched();
-		buf += PAGE_SIZE;
-		size -= PAGE_SIZE;
-	} while (size);
-
-	return size;
-out_up:
-	up_read(&mm->mmap_sem);
-	return size;
-}
-
 static ssize_t read_zero(struct file * file, char __user * buf, 
 			 size_t count, loff_t *ppos)
 {
-	unsigned long left, unwritten, written = 0;
+	size_t written;
 
 	if (!count)
 		return 0;
@@ -691,69 +636,33 @@ static ssize_t read_zero(struct file * file, char __user * buf,
 	if (!access_ok(VERIFY_WRITE, buf, count))
 		return -EFAULT;
 
-	left = count;
-
-	/* do we want to be clever? Arbitrary cut-off */
-	if (count >= PAGE_SIZE*4) {
-		unsigned long partial;
+	written = 0;
+	while (count) {
+		unsigned long unwritten;
+		size_t chunk = count;
 
-		/* How much left of the page? */
-		partial = (PAGE_SIZE-1) & -(unsigned long) buf;
-		unwritten = clear_user(buf, partial);
-		written = partial - unwritten;
-		if (unwritten)
-			goto out;
-		left -= partial;
-		buf += partial;
-		unwritten = read_zero_pagealigned(buf, left & PAGE_MASK);
-		written += (left & PAGE_MASK) - unwritten;
+		if (chunk > PAGE_SIZE)
+			chunk = PAGE_SIZE;	/* Just for latency reasons */
+		unwritten = clear_user(buf, chunk);
+		written += chunk - unwritten;
 		if (unwritten)
-			goto out;
-		buf += left & PAGE_MASK;
-		left &= ~PAGE_MASK;
-	}
-	unwritten = clear_user(buf, left);
-	written += left - unwritten;
-out:
-	return written ? written : -EFAULT;
-}
-
-static int mmap_zero(struct file * file, struct vm_area_struct * vma)
-{
-	int err;
-
-	if (vma->vm_flags & VM_SHARED)
-		return shmem_zero_setup(vma);
-	err = zeromap_page_range(vma, vma->vm_start,
-			vma->vm_end - vma->vm_start, vma->vm_page_prot);
-	BUG_ON(err == -EEXIST);
-	return err;
-}
-#else /* CONFIG_MMU */
-static ssize_t read_zero(struct file * file, char * buf, 
-			 size_t count, loff_t *ppos)
-{
-	size_t todo = count;
-
-	while (todo) {
-		size_t chunk = todo;
-
-		if (chunk > 4096)
-			chunk = 4096;	/* Just for latency reasons */
-		if (clear_user(buf, chunk))
-			return -EFAULT;
+			break;
 		buf += chunk;
-		todo -= chunk;
+		count -= chunk;
 		cond_resched();
 	}
-	return count;
+	return written ? written : -EFAULT;
 }
 
 static int mmap_zero(struct file * file, struct vm_area_struct * vma)
 {
+#ifndef CONFIG_MMU
 	return -ENOSYS;
+#endif
+	if (vma->vm_flags & VM_SHARED)
+		return shmem_zero_setup(vma);
+	return 0;
 }
-#endif /* CONFIG_MMU */
 
 static ssize_t write_full(struct file * file, const char __user * buf,
 			  size_t count, loff_t *ppos)