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

[Alessio Balsini]

On Wed, Feb 17, 2021 at 03:05:07PM +0100, Miklos Szeredi wrote:
> On Mon, Jan 25, 2021 at 4:31 PM Alessio Balsini <balsini@xxxxxxxxxxx> 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@xxxxxxx/
> >
> > Signed-off-by: Alessio Balsini <balsini@xxxxxxxxxxx>
> > ---
> >  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

Hi Miklos,

I don't have a strong preference for this and will drop the ctime/atime
updates in v13.


For the records, here follows my reasoning for which I decided to update
atime/ctime here.

>From the stats(2) man it just says that it's not guaranteed that atime
would be updated, as `Other routines, like mmap(2), may or may not
update st_atime.`

Something similar according to the inotify(7) man that warns not to trigger events
after mmap(2), msync(2), and munmap(2) operations.

The mmap(2) man mentions that st_ctime and st_mtime would be updated for
file mappings with PROT_WRITE and MAP_SHARED, before a msync(2) with
MS_SYNC or MS_ASYNC.
This passthrough scenario is slightly different from the standard mmap,
but it seems to me that we are kind of falling into a similar use case
for the atime/ctime update.
I would imagine this is why OverlayFS updates atime/ctime too in
ovl_mmap(), through ovl_copyattr().

Thanks,
Alessio