Re: [Virtio-fs] [PATCH v2 7/9] virtiofsd: Add inodes_by_handle hash table

[Max Reitz]

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 <mreitz@redhat.com>
> > Reviewed-by: Connor Kuehl <ckuehl@redhat.com>
> > ---
> >   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
> >
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