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Subject[PATCH 48/73] union-mount: Add wrapper for lookup_union_locked() and RCU hook [ver #2]
Date
Add a wrapper function for lookup_union_locked() that locks the parent
directory and follows the mount after lookup. This is appropriate for calling
from do_lookup() when in refwalk mode.

Also add an RCU-mode pathwalk lookup function. This need not leave RCU-mode if
the upper dentry is appropriately assembled or the lower dentry can be validly
used.

Original-author: Valerie Aurora <vaurora@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com> (Further development)
---

fs/namei.c | 149 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
1 files changed, 147 insertions(+), 2 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 2d69ce1..c0adf4c 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1195,6 +1195,9 @@ static int __lookup_union(struct nameidata *nd, struct qstr *name,
* layer's directory to the union stack for the topmost
* directory.
*/
+#warning what if the directory is managed?
+#warning should we d_revalidate the lower dentry?
+#warning how to handle automounts?
follow_mount(&lower);

if (!topmost->dentry->d_inode) {
@@ -1277,6 +1280,144 @@ static int lookup_union_locked(struct nameidata *nd, struct qstr *name,
}

/*
+ * lookup_union - union mount-aware part of do_lookup()
+ *
+ * do_lookup()-style wrapper for lookup_union(). Follows mounts.
+ */
+static int lookup_union(struct nameidata *nd, struct qstr *name,
+ struct path *topmost)
+{
+ struct dentry *parent = nd->path.dentry;
+ struct inode *dir = parent->d_inode;
+ int err;
+
+ mutex_lock(&dir->i_mutex);
+ err = lookup_union_locked(nd, name, topmost);
+ mutex_unlock(&dir->i_mutex);
+ if (err)
+ return err;
+
+ return follow_managed(topmost, nd->flags);
+}
+
+/*
+ * lookup_union_rcu - Handle union mounted dentries in RCU-walk mode
+ * @nd: The current pathwalk state (refers to @parent currently)
+ * @parent: The parent directory (holds the union stack)
+ * @path: The point just looked up in @parent
+ * @parent_seq: The d_seq of @parent at the point of lookup
+ * @inode: The inode at @dentry (*@inode is NULL if negative dentry)
+ *
+ * Handle a dentry that represents a non-directory file or a hole/reference in
+ * a union mount upperfs. This involves transiting to the lower file, provided
+ * we aren't going to open the lower file for writing - otherwise we have to
+ * copy the file up (which we can't do in rcuwalk mode).
+ *
+ * Directories are handled differently: they're unconditionally and completely
+ * mirrored from the lowerfs to the upperfs as soon as we encounter them in a
+ * lookup. However, since we don't create dentries in rcuwalk mode, this will
+ * be handled automatically by refwalk mode.
+ *
+ * We return true if we don't need to do anything or if we've successfully
+ * updated the path. If we need to drop out of RCU-walk and go to refwalk
+ * mode, we return false.
+ */
+static bool lookup_union_rcu(struct nameidata *nd,
+ struct dentry *parent,
+ struct path *path,
+ unsigned parent_seq,
+ struct inode **inode)
+{
+ struct dentry *dentry = path->dentry;
+ struct inode *parent_inode = nd->inode;
+ unsigned layer, layers;
+
+ /* Handle non-unionmount dentries first. The union stack will have
+ * been built during the initial lookup of the parent dir, so if it's
+ * not there, it's not unioned.
+ */
+ if (!IS_DIR_UNIONED(parent))
+ return true;
+
+ /* If it's positive then no further lookup is needed: the file or
+ * directory has been copied up and the user gets to play with that.
+ */
+ if (*inode)
+ return true;
+
+ /* If this dentry is a blocker, then stop here. */
+ if (d_is_whiteout(dentry) ||
+ (IS_OPAQUE(parent_inode) && !d_is_fallthru(dentry)))
+ return true;
+
+ /* At this point we have a negative dentry in the unionmount that may
+ * be overlaying a non-directory file in a lower filesystem, so we loop
+ * through the union stack of the parent directory to try to find a
+ * usable dentry further down.
+ */
+ layers = parent->d_sb->s_union_count;
+ for (layer = 0; layer < layers; layer++) {
+ /* Look for the a matching dentry in this layer, assuming it's
+ * still valid. Since the lower fs is hard locked R/O,
+ * revalidation ought to be unnecessary.
+ */
+ unsigned ldseq, seq;
+ struct dentry *lower_dir, *lower;
+ struct path *lower_path = union_find_dir(parent, layer);
+ if (!lower_path->mnt)
+ continue;
+
+ lower_dir = lower_path->dentry;
+ ldseq = read_seqcount_begin(&lower_dir->d_seq);
+
+ if (unlikely(lower_dir->d_flags & DCACHE_OP_REVALIDATE)) {
+ if (unlikely(d_revalidate(lower_dir, nd) <= 0) ||
+ __read_seqcount_retry(&lower_dir->d_seq, ldseq))
+ return false;
+ }
+
+ lower = __d_lookup_rcu(lower_dir, &dentry->d_name, &seq, inode);
+ if (!lower)
+ return false;
+
+ /* We've got a negative dentry which can mean several things: a
+ * plain negative dentry is ignored and lookup continues to the
+ * next layer; but a whiteout or a non-fallthru in an opaque
+ * dir covers everything below it.
+ */
+ if (!*inode) {
+ if (d_is_whiteout(lower) ||
+ (IS_OPAQUE(parent_inode) && !d_is_fallthru(lower))) {
+ if (read_seqcount_retry(&lower_dir->d_seq,
+ ldseq))
+ return false;
+ return true;
+ }
+ continue;
+ }
+
+ /* If the lower dentry is a directory then it will need copying
+ * up before we can make use of it.
+ */
+ if (S_ISDIR((*inode)->i_mode))
+ return false;
+
+ /* We have a file in a lower fs that we can use */
+ if (read_seqcount_retry(&lower_dir->d_seq, ldseq) ||
+ __read_seqcount_retry(&parent->d_seq, parent_seq))
+ return false;
+
+ path->mnt = lower_path->mnt;
+ path->dentry = lower;
+ nd->seq = seq;
+ return true;
+ }
+
+ /* Found nothing, so just use the top negative dentry */
+ return dentry;
+}
+
+/*
* Allocate a dentry with name and parent, and perform a parent
* directory ->lookup on it. Returns the new dentry, or ERR_PTR
* on error. parent->d_inode->i_mutex must be held. d_lookup must
@@ -1351,14 +1492,15 @@ static int do_lookup(struct nameidata *nd, struct qstr *name,
* do the non-racy lookup, below.
*/
if (nd->flags & LOOKUP_RCU) {
- unsigned seq;
+ unsigned seq, pseq;
*inode = nd->inode;
dentry = __d_lookup_rcu(parent, name, &seq, inode);
if (!dentry)
goto unlazy;

/* Memory barrier in read_seqcount_begin of child is enough */
- if (__read_seqcount_retry(&parent->d_seq, nd->seq))
+ pseq = nd->seq;
+ if (__read_seqcount_retry(&parent->d_seq, pseq))
return -ECHILD;
nd->seq = seq;

@@ -1372,8 +1514,11 @@ static int do_lookup(struct nameidata *nd, struct qstr *name,
}
if (unlikely(d_need_lookup(dentry)))
goto unlazy;
+
path->mnt = mnt;
path->dentry = dentry;
+ if (unlikely(!lookup_union_rcu(nd, parent, path, pseq, inode)))
+ goto unlazy;
if (unlikely(!__follow_mount_rcu(nd, path, inode)))
goto unlazy;
if (unlikely(path->dentry->d_flags & DCACHE_NEED_AUTOMOUNT))


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