On Tue, Aug 10, 2021 at 04:13:44PM +0200, Hanna Reitz wrote: > On 10.08.21 16:07, Vivek Goyal wrote: > > On Mon, Aug 09, 2021 at 06:47:18PM +0200, Hanna Reitz wrote: > > > On 09.08.21 18:10, Vivek Goyal wrote: > > > > On Fri, Jul 30, 2021 at 05:01:32PM +0200, 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> > > > > > --- > > > > > tools/virtiofsd/passthrough_ll.c | 81 > > > > > +++++++++++++++++++++++++------- > > > > > 1 file changed, 65 insertions(+), 16 deletions(-) > > > > > > > > > > diff --git a/tools/virtiofsd/passthrough_ll.c > > > > > b/tools/virtiofsd/passthrough_ll.c > > > > > index 487448d666..f9d8b2f134 100644 > > > > > --- a/tools/virtiofsd/passthrough_ll.c > > > > > +++ b/tools/virtiofsd/passthrough_ll.c > > > > > @@ -180,7 +180,8 @@ struct lo_data { > > > > > int announce_submounts; > > > > > bool use_statx; > > > > > struct lo_inode root; > > > > > - GHashTable *inodes; /* protected by lo->mutex */ > > > > > + GHashTable *inodes_by_ids; /* protected by lo->mutex */ > > > > > + GHashTable *inodes_by_handle; /* protected by lo->mutex */ > > > > > struct lo_map ino_map; /* protected by lo->mutex */ > > > > > struct lo_map dirp_map; /* protected by lo->mutex */ > > > > > struct lo_map fd_map; /* protected by lo->mutex */ > > > > > @@ -263,8 +264,9 @@ static struct { > > > > > /* That we loaded cap-ng in the current thread from the saved */ > > > > > static __thread bool cap_loaded = 0; > > > > > -static struct lo_inode *lo_find(struct lo_data *lo, struct stat *st, > > > > > - uint64_t mnt_id); > > > > > +static struct lo_inode *lo_find(struct lo_data *lo, > > > > > + const struct lo_fhandle *fhandle, > > > > > + struct stat *st, uint64_t mnt_id); > > > > > static int xattr_map_client(const struct lo_data *lo, const char > > > > > *client_name, > > > > > char **out_name); > > > > > @@ -1064,18 +1066,40 @@ out_err: > > > > > fuse_reply_err(req, saverr); > > > > > } > > > > > -static struct lo_inode *lo_find(struct lo_data *lo, struct stat *st, > > > > > - uint64_t mnt_id) > > > > > +static struct lo_inode *lo_find(struct lo_data *lo, > > > > > + const struct lo_fhandle *fhandle, > > > > > + struct stat *st, uint64_t mnt_id) > > > > > { > > > > > - struct lo_inode *p; > > > > > - struct lo_key key = { > > > > > + struct lo_inode *p = NULL; > > > > > + struct lo_key ids_key = { > > > > > .ino = st->st_ino, > > > > > .dev = st->st_dev, > > > > > .mnt_id = mnt_id, > > > > > }; > > > > > pthread_mutex_lock(&lo->mutex); > > > > > - p = g_hash_table_lookup(lo->inodes, &key); > > > > > + if (fhandle) { > > > > > + p = g_hash_table_lookup(lo->inodes_by_handle, fhandle); > > > > > + } > > > > > + if (!p) { > > > > > + p = g_hash_table_lookup(lo->inodes_by_ids, &ids_key); > > > > So even if fhandle is not NULL, we will still lookup the inode > > > > object in lo->inodes_by_ids? I thought fallback was only required > > > > if we could not generate file handle to begin with and in that case > > > > fhandle will be NULL? > > > Well. I think it depends again on when file handle generation can fail > > > and > > > when it cannot. If we assume it can randomly fail at any time, then it’s > > > possible we create an lo_inode with an O_PATH fd, but later we are able to > > > generate a file handle for it. So we first try a lookup by file handle > > > here, which would fail, but we’d still have to try a lookup by IDs, so we > > > can find the O_PATH lo_inode. > > > > > > An example case would be if at first we weren’t able to open a mount fd > > > (because this file is a device node and the first lo_inode looked up on > > > its > > > filesystem), and so we couldn’t generate a file handle that we would be > > > sure > > > would work; but later for the lookup we can generate a file handle > > > (because > > > some other node on that filesystem has been opened by then, so we have a > > > mount fd). > > Ok, got it. If we are assuming that file handle generation can fail > > randomly, then what will happen in following scenario. > > > > - lookup, file handle generated, inode added to both hash tables. > > > > - another lookup, handle generation failed. We call lo_find(), it > > finds inode in lo->inodes_by_ids but rejects it because p->fd == -1. > > > > - Now lo_find() will return NULL and caller will assume inode could > > not be found (despite the fact it is in there) and caller lo_do_lookup() > > will try to add new inode to hash tables. So we will have two inode > > instances in hash table with same st_dev, st_ino, mnt_id. One will > > have file handle while other will have O_PATH fd. > > > > So we have two inodes in cache representing same file. One using file > > handle while other using O_PATH fd. > > > > One side affect of this is says guest has looked up a file (and got > > node id 1, fhandle based inode). And later guest is revalidating > > that inode, this time it could get inode 2 (O_PATH fd). Guest will > > think inode has changed and discard previous inode and trigger > > another lookup. This typically happens only if file has gone away. > > But now it will happen because we have two inodes in cache representing > > same file. > > > > There might be other cases where this is bad. I can't think of any > > at this point of time. > > > > If could solve the issue of mount_fd, then we have to use fallback > > path probably only for EOPNOTSUPP case. And then we can be sure > > that cache will always have one inode either fhandle based or > > O_PATH based (and not both). > > OK, but can we truly solve the mount_fd issue? > > What I think we could do is have two variants of the file handle generation > function, one which is supposed to create a usable file handle (so this > version will ensure mount_fds contains a valid fd for the mount ID), and one > that just generates a file handle for lookup (i.e. it doesn’t look into > mount_fds at all). The latter version would practically only fail in the > EOPNOTSUPP case. > > Would that get around the issue?
IIUC, suggestion is that in lo_do_lookup() we will use first variant and in lookup_rename() we will use second variant. If yes, that does not solve the issue of having two inodes representing same file. lo_do_lookup() might be successful first time and add inode with fhandle and fail next time and add a new inode with O_PATH fd. Maybe this will not happen easily because first operation will add mount_fd and then second operation will find existing mount_fd and will not fail atleast due to mount_fd. Might fail due to some other temporary resource failure etc. Vivek