On Wed, Oct 20, 2021 at 12:02:32PM +0200, Hanna Reitz wrote: > On 19.10.21 22:02, Vivek Goyal wrote: > > On Thu, Sep 16, 2021 at 10:40:43AM +0200, Hanna 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: Hanna Reitz <hre...@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 bd8fc922ea..b7d6aa7f9d 100644 > > > --- a/tools/virtiofsd/passthrough_ll.c > > > +++ b/tools/virtiofsd/passthrough_ll.c > > > @@ -186,7 +186,8 @@ 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 */ > > > @@ -275,8 +276,9 @@ 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); > > > @@ -1143,18 +1145,40 @@ 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); > > > + /* > > > + * When we had to fall back to looking up an inode by its > > > + * inode ID, ensure that we hit an entry that has a valid file > > > + * descriptor. Having an FD open means that the inode cannot > > > + * really be deleted until the FD is closed, so that the inode > > > + * ID remains valid until we evict our lo_inode. > > > + * With no FD open (and just a file handle), the inode can be > > > + * deleted while we still have our lo_inode, and so the inode > > > + * ID may be reused by a completely different new inode. We > > > + * then must look up the lo_inode by file handle, because this > > > + * handle contains a generation ID to differentiate between > > > + * the old and the new inode. > > > + */ > > > + if (p && p->fd == -1) { > > > + p = NULL; > > > + } > > What happens in following scenario. > > > > - Say I have a hard linked file foo.txt with link foo-link.txt. > > > > - I lookup foo.txt. We generate file handle and add inode for foo.txt > > to inode cache. lo_inode->fhandle will be valie but lo_inode->fd == -1. > > > > - Now later lookup for foo-link.txt happens. Say this time we can't > > generate file handle. > > Which we’ve already decided is practically impossible.
Agreed that probably is very less but it can happen when sufficient resources are not available, like -ENOMEM. static long do_sys_name_to_handle(struct path *path, struct file_handle __user *ufh, int __user *mnt_id) { handle = kmalloc(sizeof(struct file_handle) + f_handle.handle_bytes, GFP_KERNEL); if (!handle) return -ENOMEM; } > > > When we try to lookup inode, lo_find() should > > return NULL. It will find inode by ids but not use it because inode > > was added using file handle and p->fd == -1. That means lookup > > for foo-link.txt will end up adding another inode, when it should > > not have? > > Yes, it would end up adding another inode, which doesn’t seem catastrophic > to me. > But again, the whole case seems impossible to me. Given we can get -ENOMEM error it is not impossible. I thought all along you wanted to write code so that we could fallback to ids in case of errors. Anyway, if you agree that except the case of -EOPNOTSUPP, we don't have to worry about fallback, then let us just reutrn error to caller if get_file_handle() fails (except the case of -EOPNOTSUPP). And then lo_find() logic could be simpler too. And there is no need for checks like this. if (p && p->fd == -1) { p = NULL; } if (fhandle) { lookup_using_handle } else { lookup_using_ids } Thanks Vivek