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Subject[PATCH v2 1/2] anon_inodes: allow external inode allocations
Date
DRM core shares a single address_space across all inodes that belong to
the same DRM device. This allows efficient unmapping of user-space
mappings during buffer eviction. However, there is no easy way to get a
shared address_space for DRM devices during initialization. Therefore, we
currently delay this step until the first ->open() and save the given
inode for future use.

This causes ugly delayed initialization throughout the DRM core. TTM
devices end up without a dev_mapping pointer and we have to carefully
respect any underlying filesystem implementation so we don't corrupt the
inode->i_mapping and inode->i_data fields.

We can avoid this if we were allowed to allocate an anonymous inode for
each DRM device. We only have to set file->f_mapping during ->open()
and no longer need to adjust inode mappings. As fs/anon_inodes.c already
provides a minimal internal FS mount, we extend it to also provide
anonymous inodes for use in drivers like DRM.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Wanted-by: Daniel Vetter <daniel.vetter@ffwll.ch>
---
Hi Linus & Al

Any chance I could get an ACK on this? It's fairly trivial and allows us to
simplify the DRM logic quite a bit (see patch 2/2 and I have 2 more DRM cleanup
patches pending). This has been pending on fsdevel for 2 months now and all we
need is an ACK from an VFS maintainer.

I ditched the idea to allocate "struct address_space" separately in DRM as akpm
told me that this might get messy without an underlying inode. So I think the
anon-inode approach is the way to go.

Thanks
David

fs/anon_inodes.c | 36 +++++++++++++++++++++++++++++-------
include/linux/anon_inodes.h | 1 +
2 files changed, 30 insertions(+), 7 deletions(-)

diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
index 85c9618..35263a3 100644
--- a/fs/anon_inodes.c
+++ b/fs/anon_inodes.c
@@ -25,6 +25,7 @@
static struct vfsmount *anon_inode_mnt __read_mostly;
static struct inode *anon_inode_inode;
static const struct file_operations anon_inode_fops;
+static struct dentry *anon_inode_root;

/*
* anon_inodefs_dname() is called from d_path().
@@ -87,19 +88,18 @@ static struct inode *anon_inode_mkinode(struct super_block *s)
static struct dentry *anon_inodefs_mount(struct file_system_type *fs_type,
int flags, const char *dev_name, void *data)
{
- struct dentry *root;
- root = mount_pseudo(fs_type, "anon_inode:", NULL,
+ anon_inode_root = mount_pseudo(fs_type, "anon_inode:", NULL,
&anon_inodefs_dentry_operations, ANON_INODE_FS_MAGIC);
- if (!IS_ERR(root)) {
- struct super_block *s = root->d_sb;
+ if (!IS_ERR(anon_inode_root)) {
+ struct super_block *s = anon_inode_root->d_sb;
anon_inode_inode = anon_inode_mkinode(s);
if (IS_ERR(anon_inode_inode)) {
- dput(root);
+ dput(anon_inode_root);
deactivate_locked_super(s);
- root = ERR_CAST(anon_inode_inode);
+ anon_inode_root = ERR_CAST(anon_inode_inode);
}
}
- return root;
+ return anon_inode_root;
}

static struct file_system_type anon_inode_fs_type = {
@@ -285,6 +285,28 @@ err_put_unused_fd:
}
EXPORT_SYMBOL_GPL(anon_inode_getfd);

+/**
+ * anon_inode_new - create private anonymous inode
+ *
+ * Creates a new inode on the anonymous inode FS for driver's use. The inode has
+ * it's own address_space compared to the shared anon_inode_inode. It can be
+ * used in situations where user-space mappings have to be shared across
+ * different files but no backing inode is available.
+ *
+ * Call iput(inode) to release the inode.
+ *
+ * RETURNS:
+ * New inode on success, error pointer on failure.
+ */
+struct inode *anon_inode_new(void)
+{
+ if (IS_ERR(anon_inode_root))
+ return ERR_CAST(anon_inode_root);
+
+ return anon_inode_mkinode(anon_inode_root->d_sb);
+}
+EXPORT_SYMBOL_GPL(anon_inode_new);
+
static int __init anon_inode_init(void)
{
int error;
diff --git a/include/linux/anon_inodes.h b/include/linux/anon_inodes.h
index cf573c2..d17378c 100644
--- a/include/linux/anon_inodes.h
+++ b/include/linux/anon_inodes.h
@@ -18,6 +18,7 @@ struct file *anon_inode_getfile_private(const char *name,
void *priv, int flags);
int anon_inode_getfd(const char *name, const struct file_operations *fops,
void *priv, int flags);
+struct inode *anon_inode_new(void);

#endif /* _LINUX_ANON_INODES_H */

--
1.8.4


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