Re: [RFC][PATCH 4/4] ovl: allow concurrent copy up data of different files

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, Nov 15, 2016 at 09:20:32PM +0200, Amir Goldstein wrote:
> On Tue, Nov 15, 2016 at 5:56 PM, Vivek Goyal <vgoyal@redhat.com> wrote:
> > On Sat, Nov 12, 2016 at 09:36:04PM +0200, Amir Goldstein wrote:
> >> Currently, all copy operations are serialized by taking the
> >> lock_rename(workdir, upperdir) locks for the entire copy up
> >> operation, including the data copy up.
> >>
> >> Copying up data may take a long time with large files and
> >> holding s_vfs_rename_mutex during that time blocks all
> >> rename and copy up operations on the entire underlying fs.
> >>
> >> This change addresses this problem by changing the copy up
> >> locking scheme for different types of files as follows.
> >>
> >> Directories:
> >>   <maybe> inode_lock(ovl_inode)
> >
> > Hi Amir,
> >
> > What does <maybe> mean here? Is it optional?
> >
> 
> 
> It means that some execution paths inode_lock(ovl_inode) is taken (e.g. rmdir()
> of overlay dir) and in some execution paths it is not taken (e.g. when
> copying up
> the victim inodes' parents).
> 
> What I have not explain properly is that my change does not add any new
> inode_lock(ovl_inode) calls for directories and special files - it is taken in
> VFS before getting to overlay code.
> I listed the inode_lock(ovl_inode) in the locking scheme of directories and
> special files to show that is safe (locking order wise) to take the same lock
> inside overlay code, for regular files on open.
> 

Ok, got it. Only in case of regular files (non-zero size), we have added
the inode_lock(ovl_inode) and that's required because we will be dropping
lock_rename() locks temporarily for data copy and want to make sure
another thread does not trigger a parallel copy up of same file.

Thanks
Vivek

> >>     lock_rename(workdir, upperdir)
> >>       copy up dir to workdir
> >>       move dir to upperdir
> >>
> >> Special and zero size files:
> >>   inode_lock(ovl_inode)
> >>     lock_rename(workdir, upperdir)
> >>       copy up file to workdir (no data)
> >>       move file to upperdir
> >>
> >
> > If we are copying up directories and special and zero file under
> > lock_rename() and don't drop it during the whole operation, then
> > why do we need to take ovl_inode lock.
> >
> 
> So we really don't take it, but for all the call sites of ovl_copy_up()
> except for the ovl_d_real() for regular files open, the ovl_inode lock is
> already taken in VFS.
> 
> > IOW, current code seems to be just doing lock_rename(workdir, upperdir)
> > for the copy up operation. But now this new code also requires
> > inode_lock(ovl_inode) and I am trying to understand why.
> >
> 
> So inode_lock(ovl_inode) is now taken ALSO in the only path it was not
> already taken until now. And the reason that we take it is so we can release
> lock_rename() for the duration of copy up data.
> 
> Hope I was able to clarify myself.
> 
> Amir.
> 
> > Thanks
> > Vivek
> >
> >> Regular files with data:
> >>   inode_lock(ovl_inode)
> >>     lock_rename(workdir, upperdir)
> >>       copy up file to workdir (no data)
> >>     unlock_rename(workdir, upperdir)
> >>       copy data to file in workdir
> >>     lock_rename(workdir, upperdir)
> >>       move file to upperdir
> >>
> >> Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> >> ---
> >>  fs/overlayfs/copy_up.c | 70 +++++++++++++++++++++++++++++++++++++++++++++++---
> >>  fs/overlayfs/inode.c   |  3 +++
> >>  2 files changed, 69 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> >> index a16127b..1b9705e 100644
> >> --- a/fs/overlayfs/copy_up.c
> >> +++ b/fs/overlayfs/copy_up.c
> >> @@ -230,6 +230,44 @@ int ovl_set_attr(struct dentry *upperdentry, struct kstat *stat)
> >>       return err;
> >>  }
> >>
> >> +/*
> >> + * Called with dentry->d_inode lock held only for the last (leaf) copy up
> >> + * from __ovl_copy_up(), so it is NOT held when called for ancestor
> >> + * directory from __ovl_copy_up()
> >> + *
> >> + * Called with lock_rename(workdir, upperdir) locks held.
> >> + *
> >> + * lock_rename() locks remain locked throughout the copy up
> >> + * of non regular files and zero sized regular files.
> >> + *
> >> + * lock_rename() locks are released during ovl_copy_up_data()
> >> + * of non zero sized regular files. During this time, the overlay
> >> + * dentry->d_inode lock is still held to avoid concurrent
> >> + * copy up of files with data.
> >> + *
> >> + * Maybe a better description of this locking scheme:
> >> + *
> >> + * Directories:
> >> + *   <maybe> inode_lock(ovl_inode)
> >> + *     lock_rename(workdir, upperdir)
> >> + *       copy up dir to workdir
> >> + *       move dir to upperdir
> >> + *
> >> + * Special and zero size files:
> >> + *   inode_lock(ovl_inode)
> >> + *     lock_rename(workdir, upperdir)
> >> + *       copy up file to workdir (no data)
> >> + *       move file to upperdir
> >> + *
> >> + * Regular files with data:
> >> + *  inode_lock(ovl_inode)
> >> + *    lock_rename(workdir, upperdir)
> >> + *      copy up file to workdir (no data)
> >> + *    unlock_rename(workdir, upperdir)
> >> + *      copy data to file in workdir
> >> + *    lock_rename(workdir, upperdir)
> >> + *      move file to upperdir
> >> + */
> >>  static int ovl_copy_up_locked(struct dentry *workdir, struct dentry *upperdir,
> >>                             struct dentry *dentry, struct path *lowerpath,
> >>                             struct kstat *stat, const char *link)
> >> @@ -274,16 +312,39 @@ static int ovl_copy_up_locked(struct dentry *workdir, struct dentry *upperdir,
> >>       if (err)
> >>               goto out2;
> >>
> >> -     if (S_ISREG(stat->mode)) {
> >> +     if (S_ISREG(stat->mode) && stat->size > 0) {
> >>               struct path upperpath;
> >>
> >>               ovl_path_upper(dentry, &upperpath);
> >>               BUG_ON(upperpath.dentry != NULL);
> >>               upperpath.dentry = newdentry;
> >>
> >> +             /*
> >> +              * Release rename locks, because copy data may take a long time,
> >> +              * and holding s_vfs_rename_mutex will block all rename and
> >> +              * copy up operations on the entire underlying fs.
> >> +              * We still hold the overlay inode lock to avoid concurrent
> >> +              * copy up of this file.
> >> +              */
> >> +             unlock_rename(workdir, upperdir);
> >> +
> >>               err = ovl_copy_up_data(lowerpath, &upperpath, stat->size);
> >> +
> >> +             /*
> >> +              * Re-aquire rename locks, before moving copied file into place.
> >> +              */
> >> +             if (unlikely(lock_rename(workdir, upperdir) != NULL)) {
> >> +                     pr_err("overlayfs: failed to re-aquire lock_rename\n");
> >> +                     err = -EIO;
> >> +             }
> >> +
> >>               if (err)
> >>                       goto out_cleanup;
> >> +
> >> +             if (WARN_ON(ovl_dentry_is_upper(dentry))) {
> >> +                     /* Raced with another copy-up? This shouldn't happen */
> >> +                     goto out_cleanup;
> >> +             }
> >>       }
> >>
> >>       err = ovl_copy_xattr(lowerpath->dentry, newdentry);
> >> @@ -366,15 +427,14 @@ static int ovl_copy_up_one(struct dentry *parent, struct dentry *dentry,
> >>                       return PTR_ERR(link);
> >>       }
> >>
> >> -     err = -EIO;
> >> -     if (lock_rename(workdir, upperdir) != NULL) {
> >> +     if (unlikely(lock_rename(workdir, upperdir) != NULL)) {
> >>               pr_err("overlayfs: failed to lock workdir+upperdir\n");
> >> +             err = -EIO;
> >>               goto out_unlock;
> >>       }
> >>
> >>       if (ovl_dentry_is_upper(dentry)) {
> >>               /* Raced with another copy-up?  Nothing to do, then... */
> >> -             err = 0;
> >>               goto out_unlock;
> >>       }
> >>
> >> @@ -433,11 +493,13 @@ static int __ovl_copy_up(struct dentry *dentry, int flags)
> >>       return err;
> >>  }
> >>
> >> +/* Called with dentry->d_inode lock held */
> >>  int ovl_copy_up(struct dentry *dentry)
> >>  {
> >>       return __ovl_copy_up(dentry, 0);
> >>  }
> >>
> >> +/* Called with dentry->d_inode lock held */
> >>  int ovl_copy_up_open(struct dentry *dentry, int flags)
> >>  {
> >>       return __ovl_copy_up(dentry, flags);
> >> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> >> index 7abae00..532b0d5 100644
> >> --- a/fs/overlayfs/inode.c
> >> +++ b/fs/overlayfs/inode.c
> >> @@ -251,11 +251,14 @@ int ovl_open_maybe_copy_up(struct dentry *dentry, unsigned int file_flags)
> >>
> >>       type = ovl_path_real(dentry, &realpath);
> >>       if (ovl_open_need_copy_up(file_flags, type, realpath.dentry)) {
> >> +             /* Take the overlay inode lock to avoid concurrent copy-up */
> >> +             inode_lock(dentry->d_inode);
> >>               err = ovl_want_write(dentry);
> >>               if (!err) {
> >>                       err = ovl_copy_up_open(dentry, file_flags);
> >>                       ovl_drop_write(dentry);
> >>               }
> >> +             inode_unlock(dentry->d_inode);
> >>       }
> >>
> >>       return err;
> >> --
> >> 2.7.4
> >>
> >> --
> >> To unsubscribe from this list: send the line "unsubscribe linux-unionfs" in
> >> the body of a message to majordomo@vger.kernel.org
> >> More majordomo info at  http://vger.kernel.org/majordomo-info.html