On 8/30/22 10:30 AM, Xiubo Li wrote:
On 8/30/22 2:17 AM, Jeff Layton wrote:
On Mon, 2022-08-29 at 12:57 +0800, xiubli@xxxxxxxxxx wrote:
From: Xiubo Li <xiubli@xxxxxxxxxx>
When unlinking a file the kclient will send a unlink request to MDS
by holding the dentry reference, and then the MDS will return 2
replies,
which are unsafe reply and a deferred safe reply.
After the unsafe reply received the kernel will return and succeed
the unlink request to user space apps.
Only when the safe reply received the dentry's reference will be
released. Or the dentry will only be unhashed from dcache. But when
the open_by_handle_at() begins to open the unlinked files it will
succeed.
URL: https://tracker.ceph.com/issues/56524
Signed-off-by: Xiubo Li <xiubli@xxxxxxxxxx>
---
V2:
- If the dentry was released and inode is evicted such as by dropping
the caches, it will allocate a new dentry, which is also unhashed.
fs/ceph/export.c | 17 ++++++++++++++++-
1 file changed, 16 insertions(+), 1 deletion(-)
diff --git a/fs/ceph/export.c b/fs/ceph/export.c
index 0ebf2bd93055..5edc1d31cd79 100644
--- a/fs/ceph/export.c
+++ b/fs/ceph/export.c
@@ -182,6 +182,7 @@ struct inode *ceph_lookup_inode(struct
super_block *sb, u64 ino)
static struct dentry *__fh_to_dentry(struct super_block *sb, u64 ino)
{
struct inode *inode = __lookup_inode(sb, ino);
+ struct dentry *dentry;
int err;
if (IS_ERR(inode))
@@ -197,7 +198,21 @@ static struct dentry *__fh_to_dentry(struct
super_block *sb, u64 ino)
iput(inode);
return ERR_PTR(-ESTALE);
}
- return d_obtain_alias(inode);
+
+ /*
+ * -ESTALE if the dentry exists and is unhashed,
+ * which should be being released
+ */
+ dentry = d_find_any_alias(inode);
+ if (dentry && unlikely(d_unhashed(dentry))) {
+ dput(dentry);
+ return ERR_PTR(-ESTALE);
+ }
+
+ if (!dentry)
+ dentry = d_obtain_alias(inode);
+
+ return dentry;
}
static struct dentry *__snapfh_to_dentry(struct super_block *sb,
This looks racy.
Suppose we have 2 racing tasks calling __fh_to_dentry for the same
inode. The first one races in and doesn't find anything. d_obtain alias
creates a disconnected dentry and returns it. The next task then finds
it, sees that it's disconnected and gets back -ESTALE.
I think you may need to detect this situation in a different way.
Yeah, you're right. Locally I have another version of patch, which
will add one di->flags bit, which is "CEPH_DENTRY_IS_UNLINKING".
If the file have hard links and there are more than one alias and one
of them is being unlinked, shouldn't we make sure we will pick a
normal one here ? If so we should iterate all the alias and filter out
the being unlinked ones.
At the same time I found another issue for the "ceph_fh_to_dentry()".
That is we never check the inode->i_generation like other filesystems,
which will make sure the inode we are trying to open is the exactly
the same one saved in userspace. The inode maybe deleted and created
before this.
It seems never happening for ceph. The MDS will make sure it won't reuse
the freed ino#.
- Xiubo
Thanks
Xiubo
