Re: [PATCH RESEND V12 8/8] fuse: Introduce passthrough for mmap

[Miklos Szeredi <miklos@szeredi.hu>]

On Mon, Jan 25, 2021 at 4:31 PM Alessio Balsini <balsini@android.com> wrote:
>
> Enabling FUSE passthrough for mmap-ed operations not only affects
> performance, but has also been shown as mandatory for the correct
> functioning of FUSE passthrough.
> yanwu noticed [1] that a FUSE file with passthrough enabled may suffer
> data inconsistencies if the same file is also accessed with mmap. What
> happens is that read/write operations are directly applied to the lower
> file system (and its cache), while mmap-ed operations are affecting the
> FUSE cache.
>
> Extend the FUSE passthrough implementation to also handle memory-mapped
> FUSE file, to both fix the cache inconsistencies and extend the
> passthrough performance benefits to mmap-ed operations.
>
> [1] https://lore.kernel.org/lkml/20210119110654.11817-1-wu-yan@tcl.com/
>
> Signed-off-by: Alessio Balsini <balsini@android.com>
> ---
>  fs/fuse/file.c        |  3 +++
>  fs/fuse/fuse_i.h      |  1 +
>  fs/fuse/passthrough.c | 41 +++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 45 insertions(+)
>
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index cddada1e8bd9..e3741a94c1f9 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -2370,6 +2370,9 @@ static int fuse_file_mmap(struct file *file, struct vm_area_struct *vma)
>         if (FUSE_IS_DAX(file_inode(file)))
>                 return fuse_dax_mmap(file, vma);
>
> +       if (ff->passthrough.filp)
> +               return fuse_passthrough_mmap(file, vma);
> +
>         if (ff->open_flags & FOPEN_DIRECT_IO) {
>                 /* Can't provide the coherency needed for MAP_SHARED */
>                 if (vma->vm_flags & VM_MAYSHARE)
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index 815af1845b16..7b0d65984608 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -1244,5 +1244,6 @@ int fuse_passthrough_setup(struct fuse_conn *fc, struct fuse_file *ff,
>  void fuse_passthrough_release(struct fuse_passthrough *passthrough);
>  ssize_t fuse_passthrough_read_iter(struct kiocb *iocb, struct iov_iter *to);
>  ssize_t fuse_passthrough_write_iter(struct kiocb *iocb, struct iov_iter *from);
> +ssize_t fuse_passthrough_mmap(struct file *file, struct vm_area_struct *vma);
>
>  #endif /* _FS_FUSE_I_H */
> diff --git a/fs/fuse/passthrough.c b/fs/fuse/passthrough.c
> index 24866c5fe7e2..284979f87747 100644
> --- a/fs/fuse/passthrough.c
> +++ b/fs/fuse/passthrough.c
> @@ -135,6 +135,47 @@ ssize_t fuse_passthrough_write_iter(struct kiocb *iocb_fuse,
>         return ret;
>  }
>
> +ssize_t fuse_passthrough_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> +       int ret;
> +       const struct cred *old_cred;
> +       struct fuse_file *ff = file->private_data;
> +       struct inode *fuse_inode = file_inode(file);
> +       struct file *passthrough_filp = ff->passthrough.filp;
> +       struct inode *passthrough_inode = file_inode(passthrough_filp);
> +
> +       if (!passthrough_filp->f_op->mmap)
> +               return -ENODEV;
> +
> +       if (WARN_ON(file != vma->vm_file))
> +               return -EIO;
> +
> +       vma->vm_file = get_file(passthrough_filp);
> +
> +       old_cred = override_creds(ff->passthrough.cred);
> +       ret = call_mmap(vma->vm_file, vma);
> +       revert_creds(old_cred);
> +
> +       if (ret)
> +               fput(passthrough_filp);
> +       else
> +               fput(file);
> +
> +       if (file->f_flags & O_NOATIME)
> +               return ret;
> +
> +       if ((!timespec64_equal(&fuse_inode->i_mtime,
> +                              &passthrough_inode->i_mtime) ||
> +            !timespec64_equal(&fuse_inode->i_ctime,
> +                              &passthrough_inode->i_ctime))) {
> +               fuse_inode->i_mtime = passthrough_inode->i_mtime;
> +               fuse_inode->i_ctime = passthrough_inode->i_ctime;

Again, violation of rules.   Not sure why this is needed, mmap(2)
isn't supposed to change mtime or ctime, AFAIK.

Thanks,
Miklos