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

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On Tue, Nov 15, 2016 at 5:56 PM, Vivek Goyal <vgoyal@xxxxxxxxxx> 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.

>>     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@xxxxxxxxx>
>> ---
>>  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
>>
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