Re: [PATCH v4 3/3] FUSE: Avoid lookup in d_revalidate()

[Dharmendra Hans]

On Thu, May 5, 2022 at 2:09 AM Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
>
> On Mon, May 02, 2022 at 03:55:21PM +0530, Dharmendra Singh wrote:
> > From: Dharmendra Singh <dsingh@xxxxxxx>
> >
> > With atomic open + lookup implemented, it is possible
> > to avoid lookups in FUSE d_revalidate() for objects
> > other than directories.
> >
> > If FUSE is mounted with default permissions then this
> > optimization is not possible as we need to fetch fresh
> > inode attributes for permission check. This lookup
> > skipped in d_revalidate() can be performed  as part of
> > open call into libfuse which is made from fuse_file_open().
> > And when we return from USER SPACE with file opened and
> > fresh attributes, we can revalidate the inode.
> >
> > Signed-off-by: Dharmendra Singh <dsingh@xxxxxxx>
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> >
> > ---
> >  fs/fuse/dir.c    | 89 ++++++++++++++++++++++++++++++++++++++++++------
> >  fs/fuse/file.c   | 30 ++++++++++++++--
> >  fs/fuse/fuse_i.h | 10 +++++-
> >  fs/fuse/ioctl.c  |  2 +-
> >  4 files changed, 115 insertions(+), 16 deletions(-)
> >
> > diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> > index 6879d3a86796..1594fecc920f 100644
> > --- a/fs/fuse/dir.c
> > +++ b/fs/fuse/dir.c
> > @@ -196,6 +196,7 @@ static void fuse_lookup_init(struct fuse_conn *fc, struct fuse_args *args,
> >   * the lookup once more.  If the lookup results in the same inode,
> >   * then refresh the attributes, timeouts and mark the dentry valid.
> >   */
> > +
> >  static int fuse_dentry_revalidate(struct dentry *entry, unsigned int flags)
> >  {
> >       struct inode *inode;
> > @@ -224,6 +225,17 @@ static int fuse_dentry_revalidate(struct dentry *entry, unsigned int flags)
> >
> >               fm = get_fuse_mount(inode);
> >
> > +             /* If atomic open is supported by FUSE then use this opportunity
> > +              * (only for non-dir) to avoid this lookup and combine
> > +              * lookup + open into single call.
> > +              */
> > +             if (!fm->fc->default_permissions && fm->fc->do_atomic_open &&
> > +                 !(flags & (LOOKUP_EXCL | LOOKUP_REVAL)) &&
> > +                 (flags & LOOKUP_OPEN) && !S_ISDIR(inode->i_mode)) {
> > +                     ret = 1;
>
> So basically we think that VFS is going to do OPEN and calling
> ->revalidate() before that. So we are returning "1" to vfs saying
> dentry is valid (despite the fact that we have no idea at this
> point of time).
>
> And later when open comes we are opening and revalidating inode etc.
>
> Seriously, IMHO, all this seems very fragile and hard to understand
> and maintain code. Things can go easily wrong if even little bit
> of assumptions change in VFS.
>
> This sounds more like VFS should know about it and if VFS knows
> that filesystem supports facility where it can open + revalidate
> at the same time, it should probably call that. Something like
> ->open_revalidate() etc. That would be much more maintainable code but
> this feels like very fragile to me, IMHO.

Actually we did not want to do it this way. We understand that code is
hard to understand and  maintain.  But we found that VFS does not have
support for this kind of functionalities i.e to combine a couple of
ops into one except 'atomic open' which would have made it very easier
for File systems.
As of now I think VFS does not do much (it calls inode permissions
checks though but we are optimising only if default permissions are
not enabled) with inode till the time it opens it. Since we are
revalidating inode after opening, it detects issues if any with the
file.

>
>
>
> > +                     goto out;
> > +             }
> > +
> >               forget = fuse_alloc_forget();
> >               ret = -ENOMEM;
> >               if (!forget)
> > @@ -515,14 +527,52 @@ static int get_security_context(struct dentry *entry, umode_t mode,
> >       return err;
> >  }
> >
> > +/*
> > + * Revalidate the inode after we got fresh attributes from user space.
> > + */
> > +static int fuse_atomic_open_revalidate_inode(struct inode *reval_inode,
> > +                                          struct dentry *entry,
> > +                                          struct fuse_forget_link *forget,
> > +                                          struct fuse_entry_out *outentry,
> > +                                          u64 attr_version)
> > +{
> > +     struct fuse_inode *fi;
> > +     struct fuse_conn *fc = get_fuse_conn(reval_inode);
> > +
> > +     /* Mode should be other than directory */
> > +     BUG_ON(S_ISDIR(reval_inode->i_mode));
> > +
> > +     if (outentry->nodeid != get_node_id(reval_inode)) {
> > +             fuse_queue_forget(fc, forget, outentry->nodeid, 1);
> > +             return -ESTALE;
> > +     }
> > +     if (fuse_stale_inode(reval_inode, outentry->generation,
> > +                          &outentry->attr)) {
> > +             fuse_make_bad(reval_inode);
> > +             return -ESTALE;
> > +     }
> > +     fi = get_fuse_inode(reval_inode);
> > +     spin_lock(&fi->lock);
> > +     fi->nlookup++;
> > +     spin_unlock(&fi->lock);
> > +
> > +     forget_all_cached_acls(reval_inode);
> > +     fuse_change_attributes(reval_inode, &outentry->attr,
> > +                            entry_attr_timeout(outentry), attr_version);
> > +     fuse_change_entry_timeout(entry, outentry);
> > +     return 0;
> > +}
> > +
> > +
> >  /*
> >   * This is common function for initiating atomic operations into libfuse.
> >   * Currently being used by Atomic create(atomic lookup + create) and
> >   * Atomic open(atomic lookup + open).
> >   */
> > -static int fuse_atomic_do_common(struct inode *dir, struct dentry *entry,
> > +static int fuse_do_atomic_common(struct inode *dir, struct dentry *entry,
> >                                struct dentry **alias, struct file *file,
> > -                              unsigned int flags, umode_t mode, uint32_t opcode)
> > +                              struct inode *reval_inode, unsigned int flags,
> > +                              umode_t mode, uint32_t opcode)
> >  {
> >       int err;
> >       struct inode *inode;
> > @@ -541,6 +591,8 @@ static int fuse_atomic_do_common(struct inode *dir, struct dentry *entry,
> >       bool atomic_create = (opcode == FUSE_ATOMIC_CREATE ? true : false);
> >       bool create_op = (opcode == FUSE_CREATE ||
> >                         opcode == FUSE_ATOMIC_CREATE) ? true : false;
> > +     u64 attr_version = fuse_get_attr_version(fm->fc);
> > +
> >       if (alias)
> >               *alias = NULL;
> >
> > @@ -609,6 +661,20 @@ static int fuse_atomic_do_common(struct inode *dir, struct dentry *entry,
> >       ff->fh = outopen.fh;
> >       ff->nodeid = outentry.nodeid;
> >       ff->open_flags = outopen.open_flags;
> > +
> > +     /* Inode revalidation was bypassed previously for type other than
> > +      * directories, revalidate now as we got fresh attributes.
> > +      */
> > +     if (reval_inode) {
> > +             err = fuse_atomic_open_revalidate_inode(reval_inode, entry,
> > +                                                     forget, &outentry,
> > +                                                     attr_version);
> > +             if (err)
> > +                     goto out_free_ff;
> > +             inode = reval_inode;
> > +             kfree(forget);
> > +             goto out_finish_open;
> > +     }
> >       inode = fuse_iget(dir->i_sb, outentry.nodeid, outentry.generation,
> >                         &outentry.attr, entry_attr_timeout(&outentry), 0);
> >       if (!inode) {
> > @@ -659,6 +725,7 @@ static int fuse_atomic_do_common(struct inode *dir, struct dentry *entry,
> >       if (!atomic_create || (outopen.open_flags & FOPEN_FILE_CREATED))
> >               fuse_dir_changed(dir);
> >       err = finish_open(file, entry, generic_file_open);
> > +out_finish_open:
> >       if (err) {
> >               fi = get_fuse_inode(inode);
> >               fuse_sync_release(fi, ff, flags);
> > @@ -686,7 +753,7 @@ static int fuse_atomic_create(struct inode *dir, struct dentry *entry,
> >       if (fc->no_atomic_create)
> >               return -ENOSYS;
> >
> > -     err = fuse_atomic_do_common(dir, entry, NULL, file, flags, mode,
> > +     err = fuse_do_atomic_common(dir, entry, NULL, file, NULL, flags, mode,
> >                                   FUSE_ATOMIC_CREATE);
> >       /* If atomic create is not implemented then indicate in fc so that next
> >        * request falls back to normal create instead of going into libufse and
> > @@ -699,9 +766,10 @@ static int fuse_atomic_create(struct inode *dir, struct dentry *entry,
> >       return err;
> >  }
> >
> > -static int fuse_do_atomic_open(struct inode *dir, struct dentry *entry,
> > -                             struct dentry **alias, struct file *file,
> > -                             unsigned int flags, umode_t mode)
> > +int fuse_do_atomic_open(struct inode *dir, struct dentry *entry,
> > +                     struct dentry **alias, struct file *file,
> > +                     struct inode *reval_inode, unsigned int flags,
> > +                     umode_t mode)
> >  {
> >       int err;
> >       struct fuse_conn *fc = get_fuse_conn(dir);
> > @@ -709,8 +777,8 @@ static int fuse_do_atomic_open(struct inode *dir, struct dentry *entry,
> >       if (!fc->do_atomic_open)
> >               return -ENOSYS;
> >
> > -     err = fuse_atomic_do_common(dir, entry, alias, file, flags, mode,
> > -                                 FUSE_ATOMIC_OPEN);
> > +     err = fuse_do_atomic_common(dir, entry, alias, file, reval_inode, flags,
> > +                                 mode, FUSE_ATOMIC_OPEN);
> >       /* Atomic open imply atomic trunc as well i.e truncate should be performed
> >        * as part of atomic open call itself.
> >        */
> > @@ -718,7 +786,6 @@ static int fuse_do_atomic_open(struct inode *dir, struct dentry *entry,
> >               if (fc->do_atomic_open)
> >                       fc->atomic_o_trunc = 1;
> >       }
> > -
> >       return err;
> >  }
> >
> > @@ -738,7 +805,7 @@ static int fuse_atomic_open(struct inode *dir, struct dentry *entry,
> >
> >       if (!create) {
> >               err = fuse_do_atomic_open(dir, entry, &alias,
> > -                                       file, flags, mode);
> > +                                       file, NULL, flags, mode);
> >               res = alias;
> >               if (!err || err != -ENOSYS)
> >                       goto out_dput;
> > @@ -774,7 +841,7 @@ static int fuse_atomic_open(struct inode *dir, struct dentry *entry,
> >        * 'mknod' + 'open' requests.
> >        */
> >       if (err == -ENOSYS) {
> > -             err = fuse_atomic_do_common(dir, entry, NULL, file, flags,
> > +             err = fuse_do_atomic_common(dir, entry, NULL, file, NULL, flags,
> >                                           mode, FUSE_CREATE);
> >       }
> >       if (err == -ENOSYS) {
> > diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> > index 829094451774..2b0548163249 100644
> > --- a/fs/fuse/file.c
> > +++ b/fs/fuse/file.c
> > @@ -125,11 +125,15 @@ static void fuse_file_put(struct fuse_file *ff, bool sync, bool isdir)
> >  }
> >
> >  struct fuse_file *fuse_file_open(struct fuse_mount *fm, u64 nodeid,
> > -                              unsigned int open_flags, bool isdir)
> > +                              struct file *file, unsigned int open_flags,
> > +                              bool isdir)
> >  {
> >       struct fuse_conn *fc = fm->fc;
> >       struct fuse_file *ff;
> > +     struct dentry *dentry = NULL;
> > +     struct dentry *parent = NULL;
> >       int opcode = isdir ? FUSE_OPENDIR : FUSE_OPEN;
> > +     int ret;
> >
> >       ff = fuse_file_alloc(fm);
> >       if (!ff)
> > @@ -138,6 +142,11 @@ struct fuse_file *fuse_file_open(struct fuse_mount *fm, u64 nodeid,
> >       ff->fh = 0;
> >       /* Default for no-open */
> >       ff->open_flags = FOPEN_KEEP_CACHE | (isdir ? FOPEN_CACHE_DIR : 0);
> > +
> > +     /* For directories we already had lookup */
> > +     if (!isdir && fc->do_atomic_open && file != NULL)
> > +             goto revalidate_atomic_open;
> > +
> >       if (isdir ? !fc->no_opendir : !fc->no_open) {
> >               struct fuse_open_out outarg;
> >               int err;
> > @@ -164,12 +173,27 @@ struct fuse_file *fuse_file_open(struct fuse_mount *fm, u64 nodeid,
> >       ff->nodeid = nodeid;
> >
> >       return ff;
> > +
> > +revalidate_atomic_open:
> > +     dentry = file->f_path.dentry;
> > +     /* Get ref on parent */
> > +     parent = dget_parent(dentry);
> > +     open_flags &= ~(O_CREAT | O_EXCL | O_NOCTTY);
> > +     ret = fuse_do_atomic_open(d_inode_rcu(parent), dentry, NULL, file,
> > +                               d_inode_rcu(dentry), open_flags, 0);
> > +     dput(parent);
> > +     if (ret)
> > +             goto err_out;
> > +     ff = file->private_data;
> > +     return ff;
> > +err_out:
> > +     return ERR_PTR(ret);
> >  }
> >
> >  int fuse_do_open(struct fuse_mount *fm, u64 nodeid, struct file *file,
> >                bool isdir)
> >  {
> > -     struct fuse_file *ff = fuse_file_open(fm, nodeid, file->f_flags, isdir);
> > +     struct fuse_file *ff = fuse_file_open(fm, nodeid, file, file->f_flags, isdir);
> >
> >       if (!IS_ERR(ff))
> >               file->private_data = ff;
> > @@ -252,7 +276,7 @@ int fuse_open_common(struct inode *inode, struct file *file, bool isdir)
> >       }
> >
> >       err = fuse_do_open(fm, get_node_id(inode), file, isdir);
> > -     if (!err)
> > +     if (!err && (!fc->do_atomic_open || isdir))
> >               fuse_finish_open(inode, file);
> >
> >  out:
> > diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> > index 24793b82303f..5c83e4249b7e 100644
> > --- a/fs/fuse/fuse_i.h
> > +++ b/fs/fuse/fuse_i.h
> > @@ -1014,6 +1014,13 @@ void fuse_finish_open(struct inode *inode, struct file *file);
> >  void fuse_sync_release(struct fuse_inode *fi, struct fuse_file *ff,
> >                      unsigned int flags);
> >
> > +/**
> > + * Send atomic create + open or lookup + open
> > + */
> > +int fuse_do_atomic_open(struct inode *dir, struct dentry *entry,
> > +                     struct dentry **alias, struct file *file,
> > +                     struct inode *reval_inode, unsigned int flags,
> > +                     umode_t mode);
> >  /**
> >   * Send RELEASE or RELEASEDIR request
> >   */
> > @@ -1317,7 +1324,8 @@ int fuse_fileattr_set(struct user_namespace *mnt_userns,
> >  /* file.c */
> >
> >  struct fuse_file *fuse_file_open(struct fuse_mount *fm, u64 nodeid,
> > -                              unsigned int open_flags, bool isdir);
> > +                              struct file *file, unsigned int open_flags,
> > +                              bool isdir);
> >  void fuse_file_release(struct inode *inode, struct fuse_file *ff,
> >                      unsigned int open_flags, fl_owner_t id, bool isdir);
> >
> > diff --git a/fs/fuse/ioctl.c b/fs/fuse/ioctl.c
> > index fbc09dab1f85..63106a54ba1a 100644
> > --- a/fs/fuse/ioctl.c
> > +++ b/fs/fuse/ioctl.c
> > @@ -408,7 +408,7 @@ static struct fuse_file *fuse_priv_ioctl_prepare(struct inode *inode)
> >       if (!S_ISREG(inode->i_mode) && !isdir)
> >               return ERR_PTR(-ENOTTY);
> >
> > -     return fuse_file_open(fm, get_node_id(inode), O_RDONLY, isdir);
> > +     return fuse_file_open(fm, get_node_id(inode), NULL, O_RDONLY, isdir);
> >  }
> >
> >  static void fuse_priv_ioctl_cleanup(struct inode *inode, struct fuse_file *ff)
> > --
> > 2.17.1
> >
>