Re: [PATCH next] sysfs: fix some bin_vm_ops errors

[Hugh Dickins <hugh@veritas.com>]

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On Sun, 22 Mar 2009, Eric Biederman wrote:
> On Sun, Mar 22, 2009 at 11:33 AM, Hugh Dickins <hugh@veritas.com> wrote:
> 
> > If the file being mmap'ed is a shmem/tmpfs file, don't fail the mmap
> > on CONFIG_NUMA=y, just because that has a set_policy and get_policy.
> > They probably won't be called, and it's not a disaster if they don't
> > work on sysfs.  vm_ops->migrate?  I couldn't find any example.  Just
> > delete CONFIG_NUMA tests, easy enough to add stubs later if required.
> 
> I agree about it being easy enough to add stubs later if required.  In my case
> I deliberately added an error check instead so that the code would fail fast
> instead of failing subtlely if anyone ever did that.  And that check appears
> to have worked in your case so I would like to retain that check.

No, I didn't really hit that case (I didn't even have CONFIG_NUMA=y),
it was just one of the several misguesses I made before hitting on the
right answer.

I can understand you preferring to keep a check on this, but I don't
believe you really want to retain _that_ check: it just causes some
mmaps to fail on CONFIG_NUMA=y configurations, which succeed on
non-NUMA configurations, and work fine on non-NUMA configurations,
so long as you don't try set_policy, get_policy or migrate.

Here's a replacement version of the patch which adds those methods,
and adds a comment on the substituted vm_file as Ben suggests.


[PATCH next] sysfs: fix some bin_vm_ops errors

Commit 86c9508eb1c0ce5aa07b5cf1d36b60c54efc3d7a
"sysfs: don't block indefinitely for unmapped files" in linux-next
crashes the PowerMac G5 when X starts up.  It's caught out by the way
powerpc's pci_mmap of legacy_mem uses shmem_zero_setup(), substituting
a new vma->vm_file whose private_data no longer points to the bin_buffer
(substitution done because some versions of X crash if that mmap fails).

The fix to this is straightforward: the original vm_file is fput() in
that case, so this mmap won't block sysfs at all, so just don't switch
over to bin_vm_ops if vm_file has changed.

But more fixes made before realizing that was the problem:-

It should not be an error if bin_page_mkwrite() finds no underlying
page_mkwrite().

Check that a file already mmap'ed has the same underlying vm_ops
_before_ pointing vma->vm_ops at bin_vm_ops.

If the file being mmap'ed is a shmem/tmpfs file, don't fail the mmap
on CONFIG_NUMA=y, just because that has a set_policy and get_policy:
provide bin_set_policy, bin_get_policy and bin_migrate.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
---
I have only tested that X now starts up fine on the G5: I don't think
I've messed up the intent of your patch, but not tested it still works.

 fs/sysfs/bin.c |   89 +++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 79 insertions(+), 10 deletions(-)

--- 2.6.29-rc8-next/fs/sysfs/bin.c	2009-03-20 12:41:45.000000000 +0000
+++ linux/fs/sysfs/bin.c	2009-03-23 01:24:48.000000000 +0000
@@ -241,9 +241,12 @@ static int bin_page_mkwrite(struct vm_ar
 	struct sysfs_dirent *attr_sd = file->f_path.dentry->d_fsdata;
 	int ret;
 
-	if (!bb->vm_ops || !bb->vm_ops->page_mkwrite)
+	if (!bb->vm_ops)
 		return -EINVAL;
 
+	if (!bb->vm_ops->page_mkwrite)
+		return 0;
+
 	if (!sysfs_get_active_two(attr_sd))
 		return -EINVAL;
 
@@ -273,12 +276,78 @@ static int bin_access(struct vm_area_str
 	return ret;
 }
 
+#ifdef CONFIG_NUMA
+static int bin_set_policy(struct vm_area_struct *vma, struct mempolicy *new)
+{
+	struct file *file = vma->vm_file;
+	struct bin_buffer *bb = file->private_data;
+	struct sysfs_dirent *attr_sd = file->f_path.dentry->d_fsdata;
+	int ret;
+
+	if (!bb->vm_ops || !bb->vm_ops->set_policy)
+		return 0;
+
+	if (!sysfs_get_active_two(attr_sd))
+		return -EINVAL;
+
+	ret = bb->vm_ops->set_policy(vma, new);
+
+	sysfs_put_active_two(attr_sd);
+	return ret;
+}
+
+static struct mempolicy *bin_get_policy(struct vm_area_struct *vma,
+					unsigned long addr)
+{
+	struct file *file = vma->vm_file;
+	struct bin_buffer *bb = file->private_data;
+	struct sysfs_dirent *attr_sd = file->f_path.dentry->d_fsdata;
+	struct mempolicy *pol;
+
+	if (!bb->vm_ops || !bb->vm_ops->get_policy)
+		return vma->vm_policy;
+
+	if (!sysfs_get_active_two(attr_sd))
+		return vma->vm_policy;
+
+	pol = bb->vm_ops->get_policy(vma, addr);
+
+	sysfs_put_active_two(attr_sd);
+	return pol;
+}
+
+static int bin_migrate(struct vm_area_struct *vma, const nodemask_t *from,
+			const nodemask_t *to, unsigned long flags)
+{
+	struct file *file = vma->vm_file;
+	struct bin_buffer *bb = file->private_data;
+	struct sysfs_dirent *attr_sd = file->f_path.dentry->d_fsdata;
+	int ret;
+
+	if (!bb->vm_ops || !bb->vm_ops->migrate)
+		return 0;
+
+	if (!sysfs_get_active_two(attr_sd))
+		return 0;
+
+	ret = bb->vm_ops->migrate(vma, from, to, flags);
+
+	sysfs_put_active_two(attr_sd);
+	return ret;
+}
+#endif
+
 static struct vm_operations_struct bin_vm_ops = {
 	.open		= bin_vma_open,
 	.close		= bin_vma_close,
 	.fault		= bin_fault,
 	.page_mkwrite	= bin_page_mkwrite,
 	.access		= bin_access,
+#ifdef CONFIG_NUMA
+	.set_policy	= bin_set_policy,
+	.get_policy	= bin_get_policy,
+	.migrate	= bin_migrate,
+#endif
 };
 
 static int mmap(struct file *file, struct vm_area_struct *vma)
@@ -287,7 +356,6 @@ static int mmap(struct file *file, struc
 	struct sysfs_dirent *attr_sd = file->f_path.dentry->d_fsdata;
 	struct bin_attribute *attr = attr_sd->s_bin_attr.bin_attr;
 	struct kobject *kobj = attr_sd->s_parent->s_dir.kobj;
-	struct vm_operations_struct *vm_ops;
 	int rc;
 
 	mutex_lock(&bb->mutex);
@@ -302,24 +370,25 @@ static int mmap(struct file *file, struc
 		goto out_put;
 
 	rc = attr->mmap(kobj, attr, vma);
-	vm_ops = vma->vm_ops;
-	vma->vm_ops = &bin_vm_ops;
 	if (rc)
 		goto out_put;
 
-	rc = -EINVAL;
-	if (bb->mmapped && bb->vm_ops != vma->vm_ops)
+	/*
+	 * PowerPC's pci_mmap of legacy_mem uses shmem_zero_setup()
+	 * to satisfy versions of X which crash if the mmap fails: that
+	 * substitutes a new vm_file, and we don't then want bin_vm_ops.
+	 */
+	if (vma->vm_file != file)
 		goto out_put;
 
-#ifdef CONFIG_NUMA
 	rc = -EINVAL;
-	if (vm_ops && ((vm_ops->set_policy || vm_ops->get_policy || vm_ops->migrate)))
+	if (bb->mmapped && bb->vm_ops != vma->vm_ops)
 		goto out_put;
-#endif
 
 	rc = 0;
 	bb->mmapped = 1;
-	bb->vm_ops = vm_ops;
+	bb->vm_ops = vma->vm_ops;
+	vma->vm_ops = &bin_vm_ops;
 out_put:
 	sysfs_put_active_two(attr_sd);
 out_unlock: