On 09.06.21 17:55, Max Reitz wrote:
Currently, lo_inode.fhandle is always NULL and so always keep an O_PATH
FD in lo_inode.fd. Therefore, when the respective inode is unlinked,
its inode ID will remain in use until we drop our lo_inode (and
lo_inode_put() thus closes the FD). Therefore, lo_find() can safely use
the inode ID as an lo_inode key, because any inode with an inode ID we
find in lo_data.inodes (on the same filesystem) must be the exact same
file.
This will change when we start setting lo_inode.fhandle so we do not
have to keep an O_PATH FD open. Then, unlinking such an inode will
immediately remove it, so its ID can then be reused by newly created
files, even while the lo_inode object is still there[1].
So creating a new file can then reuse the old file's inode ID, and
looking up the new file would lead to us finding the old file's
lo_inode, which is not ideal.
Luckily, just as file handles cause this problem, they also solve it: A
file handle contains a generation ID, which changes when an inode ID is
reused, so the new file can be distinguished from the old one. So all
we need to do is to add a second map besides lo_data.inodes that maps
file handles to lo_inodes, namely lo_data.inodes_by_handle. For
clarity, lo_data.inodes is renamed to lo_data.inodes_by_ids.
Unfortunately, we cannot rely on being able to generate file handles
every time. Therefore, we still enter every lo_inode object into
inodes_by_ids, but having an entry in inodes_by_handle is optional. A
potential inodes_by_handle entry then has precedence, the inodes_by_ids
entry is just a fallback.
Note that we do not generate lo_fhandle objects yet, and so we also do
not enter anything into the inodes_by_handle map yet. Also, all lookups
skip that map. We might manually create file handles with some code
that is immediately removed by the next patch again, but that would
break the assumption in lo_find() that every lo_inode with a non-NULL
.fhandle must have an entry in inodes_by_handle and vice versa. So we
leave actually using the inodes_by_handle map for the next patch.
[1] If some application in the guest still has the file open, there is
going to be a corresponding FD mapping in lo_data.fd_map. In such a
case, the inode will only go away once every application in the guest
has closed it. The problem described only applies to cases where the
guest does not have the file open, and it is just in the dentry cache,
basically.
Signed-off-by: Max Reitz <mre...@redhat.com>
Reviewed-by: Connor Kuehl <cku...@redhat.com>
---
tools/virtiofsd/passthrough_ll.c | 80 +++++++++++++++++++++++++-------
1 file changed, 64 insertions(+), 16 deletions(-)
[...]
@@ -3931,10 +3954,34 @@ static gboolean lo_key_equal(gconstpointer a,
gconstpointer b)
return la->ino == lb->ino && la->dev == lb->dev && la->mnt_id ==
lb->mnt_id;
}
+static guint lo_fhandle_hash(gconstpointer key)
+{
+ const struct lo_fhandle *fh = key;
+ guint hash;
+ size_t i;
+
+ /* Basically g_str_hash() */
+ hash = 5381;
+ for (i = 0; i < sizeof(fh->padding); i++) {
+ hash += hash * 33 + (unsigned char)fh->padding[i];
+ }
+ hash += hash * 33 + fh->mount_id;
Just spotted: These `+=` should be `=`.
Max
+
+ return hash;
+}
