From mboxrd@z Thu Jan 1 00:00:00 1970 References: <20210609155551.44437-1-mreitz@redhat.com> <20210609155551.44437-8-mreitz@redhat.com> <20210611200459.GB767764@redhat.com> From: Max Reitz Message-ID: <9cea5642-e5ea-961f-d816-0998e52aad9f@redhat.com> Date: Wed, 16 Jun 2021 15:38:13 +0200 MIME-Version: 1.0 In-Reply-To: <20210611200459.GB767764@redhat.com> Content-Type: text/plain; charset="utf-8"; format="flowed" Content-Transfer-Encoding: 8bit Content-Language: en-US Subject: Re: [Virtio-fs] [PATCH v2 7/9] virtiofsd: Add inodes_by_handle hash table List-Id: Development discussions about virtio-fs List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Vivek Goyal Cc: virtio-fs@redhat.com, qemu-devel@nongnu.org On 11.06.21 22:04, Vivek Goyal wrote: > On Wed, Jun 09, 2021 at 05:55:49PM +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. > Hi Max, > > What are the cases where we can not rely being able to generate file > handles? I have no idea, but it’s much easier to claim we can’t than to prove that we can. I’d rather be resilient. >> 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. > If we have to keep inodes_by_ids around, then can we just add fhandle > to the lo_key. That way we can manage with single hash table and still > be able to detect if inode ID has been reused. We cannot, because I assume we cannot rely on name_to_handle_at() working every time. Therefore, maybe at one point we can generate a file handle, and at another, we cannot – we should still be able to look up the inode regardless. If the file handle were part of inodes_by_ids, then we can look up inodes only if we can generate a file handle either every time (for a given inode) or never. Or, well, I suppose we could always create two entries, one with the file handles zeroed out, and one with the file handle specified, but I wouldn’t find that very beautiful. Max >> 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 >> Reviewed-by: Connor Kuehl >> --- >> tools/virtiofsd/passthrough_ll.c | 80 +++++++++++++++++++++++++------- >> 1 file changed, 64 insertions(+), 16 deletions(-) >> >> diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c >> index e665575401..793d2c333e 100644 >> --- a/tools/virtiofsd/passthrough_ll.c >> +++ b/tools/virtiofsd/passthrough_ll.c >> @@ -179,7 +179,8 @@ 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 */ >> @@ -257,8 +258,9 @@ 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); >> >> @@ -1032,18 +1034,39 @@ 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 IDs, >> + * ensure that we hit an entry that does not have a file >> + * handle. Entries with file handles must also have a handle >> + * alt key, so if we have not found it by that handle alt key, >> + * we must have found an entry with a mismatching handle; i.e. >> + * an entry for a different file, even though it has the same >> + * inode ID. >> + * (This can happen when we look up a new file that has reused >> + * the inode ID of some previously unlinked inode for which we >> + * still have an lo_inode object.) >> + */ >> + if (p && fhandle != NULL && p->fhandle != NULL) { >> + p = NULL; >> + } >> + } >> if (p) { >> assert(p->nlookup > 0); >> p->nlookup++; >> @@ -1183,7 +1206,7 @@ static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name, >> e->attr_flags |= FUSE_ATTR_SUBMOUNT; >> } >> >> - inode = lo_find(lo, &e->attr, mnt_id); >> + inode = lo_find(lo, NULL, &e->attr, mnt_id); >> if (inode) { >> close(newfd); >> } else { >> @@ -1213,7 +1236,7 @@ static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name, >> } >> pthread_mutex_lock(&lo->mutex); >> inode->fuse_ino = lo_add_inode_mapping(req, inode); >> - g_hash_table_insert(lo->inodes, &inode->key, inode); >> + g_hash_table_insert(lo->inodes_by_ids, &inode->key, inode); >> pthread_mutex_unlock(&lo->mutex); >> } >> e->ino = inode->fuse_ino; >> @@ -1525,7 +1548,7 @@ static struct lo_inode *lookup_name(fuse_req_t req, fuse_ino_t parent, >> return NULL; >> } >> >> - return lo_find(lo, &attr, mnt_id); >> + return lo_find(lo, NULL, &attr, mnt_id); >> } >> >> static void lo_rmdir(fuse_req_t req, fuse_ino_t parent, const char *name) >> @@ -1688,7 +1711,7 @@ static void unref_inode(struct lo_data *lo, struct lo_inode *inode, uint64_t n) >> inode->nlookup -= n; >> if (!inode->nlookup) { >> lo_map_remove(&lo->ino_map, inode->fuse_ino); >> - g_hash_table_remove(lo->inodes, &inode->key); >> + g_hash_table_remove(lo->inodes_by_ids, &inode->key); >> if (lo->posix_lock) { >> if (g_hash_table_size(inode->posix_locks)) { >> fuse_log(FUSE_LOG_WARNING, "Hash table is not empty\n"); >> @@ -3388,7 +3411,7 @@ static void lo_destroy(void *userdata) >> GHashTableIter iter; >> gpointer key, value; >> >> - g_hash_table_iter_init(&iter, lo->inodes); >> + g_hash_table_iter_init(&iter, lo->inodes_by_ids); >> if (!g_hash_table_iter_next(&iter, &key, &value)) { >> break; >> } >> @@ -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; >> + >> + return hash; >> +} >> + >> +static gboolean lo_fhandle_equal(gconstpointer a, gconstpointer b) >> +{ >> + return !memcmp(a, b, sizeof(struct lo_fhandle)); >> +} >> + >> static void fuse_lo_data_cleanup(struct lo_data *lo) >> { >> - if (lo->inodes) { >> - g_hash_table_destroy(lo->inodes); >> + if (lo->inodes_by_ids) { >> + g_hash_table_destroy(lo->inodes_by_ids); >> + } >> + if (lo->inodes_by_ids) { >> + g_hash_table_destroy(lo->inodes_by_handle); >> } >> >> if (lo->root.posix_locks) { >> @@ -3990,7 +4037,8 @@ int main(int argc, char *argv[]) >> qemu_init_exec_dir(argv[0]); >> >> pthread_mutex_init(&lo.mutex, NULL); >> - lo.inodes = g_hash_table_new(lo_key_hash, lo_key_equal); >> + lo.inodes_by_ids = g_hash_table_new(lo_key_hash, lo_key_equal); >> + lo.inodes_by_handle = g_hash_table_new(lo_fhandle_hash, lo_fhandle_equal); >> lo.root.fd = -1; >> lo.root.fuse_ino = FUSE_ROOT_ID; >> lo.cache = CACHE_AUTO; >> -- >> 2.31.1 >> >> _______________________________________________ >> Virtio-fs mailing list >> Virtio-fs@redhat.com >> https://listman.redhat.com/mailman/listinfo/virtio-fs >>