Re: [PATCH] vfs, syncfs: Do not ignore return code from ->sync_fs()

[Jeff Layton <jlayton@kernel.org>]

On Wed, 2020-12-16 at 10:44 -0500, Jeff Layton wrote:
> On Wed, 2020-12-16 at 10:14 -0500, Vivek Goyal wrote:
> > On Wed, Dec 16, 2020 at 09:57:49AM -0500, Jeff Layton wrote:
> > > On Wed, 2020-12-16 at 09:38 -0500, Vivek Goyal wrote:
> > > > I see that current implementation of __sync_filesystem() ignores the
> > > > return code from ->sync_fs(). I am not sure why that's the case.
> > > > 
> > > > Ignoring ->sync_fs() return code is problematic for overlayfs where
> > > > it can return error if sync_filesystem() on upper super block failed.
> > > > That error will simply be lost and sycnfs(overlay_fd), will get
> > > > success (despite the fact it failed).
> > > > 
> > > > I am assuming that we want to continue to call __sync_blockdev()
> > > > despite the fact that there have been errors reported from
> > > > ->sync_fs(). So I wrote this simple patch which captures the
> > > > error from ->sync_fs() but continues to call __sync_blockdev()
> > > > and returns error from sync_fs() if there is one.
> > > > 
> > > > There might be some very good reasons to not capture ->sync_fs()
> > > > return code, I don't know. Hence thought of proposing this patch.
> > > > Atleast I will get to know the reason. I still need to figure
> > > > a way out how to propagate overlay sync_fs() errors to user
> > > > space.
> > > > 
> > > > Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> > > > ---
> > > >  fs/sync.c |    8 ++++++--
> > > >  1 file changed, 6 insertions(+), 2 deletions(-)
> > > > 
> > > > Index: redhat-linux/fs/sync.c
> > > > ===================================================================
> > > > --- redhat-linux.orig/fs/sync.c	2020-12-16 09:15:49.831565653 -0500
> > > > +++ redhat-linux/fs/sync.c	2020-12-16 09:23:42.499853207 -0500
> > > > @@ -30,14 +30,18 @@
> > > >   */
> > > >  static int __sync_filesystem(struct super_block *sb, int wait)
> > > >  {
> > > > +	int ret, ret2;
> > > > +
> > > >  	if (wait)
> > > >  		sync_inodes_sb(sb);
> > > >  	else
> > > >  		writeback_inodes_sb(sb, WB_REASON_SYNC);
> > > >  
> > > > 
> > > >  	if (sb->s_op->sync_fs)
> > > > -		sb->s_op->sync_fs(sb, wait);
> > > > -	return __sync_blockdev(sb->s_bdev, wait);
> > > > +		ret = sb->s_op->sync_fs(sb, wait);
> > > > +	ret2 = __sync_blockdev(sb->s_bdev, wait);
> > > > +
> > > > +	return ret ? ret : ret2;
> > > >  }
> > > >  
> > > > 
> > > >  /*
> > > > 
> > > 
> > > I posted a patchset that took a similar approach a couple of years ago,
> > > and we decided not to go with it [1].
> > > 
> > > While it's not ideal to ignore the error here, I think this is likely to
> > > break stuff.
> > 
> > So one side affect I see is that syncfs() might start returning errors
> > in some cases which were not reported at all. I am wondering will that
> > count as breakage.
> > 
> > > What may be better is to just make sync_fs void return, so
> > > people don't think that returned errors there mean anything.
> > 
> > May be. 
> > 
> > But then question remains that how do we return error to user space
> > in syncfs(fd) for overlayfs. I will not be surprised if other
> > filesystems want to return errors as well.
> > 
> > Shall I create new helpers and call these in case of syncfs(). But
> > that too will start returning new errors on syncfs(). So it does
> > not solve that problem (if it is a problem).
> > 
> > Or we can define a new super block op say ->sync_fs2() and call that
> > first and in that case capture return code. That way it will not
> > impact existing cases and overlayfs can possibly make use of
> > ->sync_fs2() and return error. IOW, impact will be limited to
> > only file systems which chose to implement ->sync_fs2().
> > 
> > Thanks
> > Vivek
> > 
> 
> Sure, it's possible to add a sb->sync_fs2, but the problem is that
> sync_fs is a superblock op, and is missing a lot of important context
> about how it got called.
> 
> syncfs(2) syscall takes a file descriptor argument. I'd add a new f_op-
> > syncfs vector and turn most of the current guts of the syncfs syscall
> into a generic_syncfs() that gets called when f_op->syncfs isn't
> defined.
> 
> Overlayfs could then add a ->syncfs op that would give it control over
> what error gets returned. With that, you could basically leave the old
> sb->sync_fs routine alone.
> 
> I think that's probably the safest approach for allowing overlayfs to
> propagate syncfs errors from the upper layer to the overlay.
> 

To be clear, I mean something like this (draft, untested) patch. You'd
also need to add a new ->syncfs op for overlayfs, and that could just do
a check_and_advance against the upper layer sb's errseq_t after calling
sync_filesystem.

-----------------------8<-------------------------

[PATCH] vfs: add new f_op->syncfs vector

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/sync.c          | 30 +++++++++++++++++++++---------
 include/linux/fs.h |  1 +
 2 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/fs/sync.c b/fs/sync.c
index 1373a610dc78..fc7f73762b9e 100644
--- a/fs/sync.c
+++ b/fs/sync.c
@@ -155,27 +155,39 @@ void emergency_sync(void)
 	}
 }
 
+static int generic_syncfs(struct file *file)
+{
+	int ret, ret2;
+	struct super_block *sb = file->f_path.dentry->d_sb;
+
+	down_read(&sb->s_umount);
+	ret = sync_filesystem(sb);
+	up_read(&sb->s_umount);
+
+	ret2 = errseq_check_and_advance(&sb->s_wb_err, &f.file->f_sb_err);
+
+	fdput(f);
+	return ret ? ret : ret2;
+}
+
 /*
  * sync a single super
  */
 SYSCALL_DEFINE1(syncfs, int, fd)
 {
 	struct fd f = fdget(fd);
-	struct super_block *sb;
-	int ret, ret2;
+	int ret;
 
 	if (!f.file)
 		return -EBADF;
-	sb = f.file->f_path.dentry->d_sb;
 
-	down_read(&sb->s_umount);
-	ret = sync_filesystem(sb);
-	up_read(&sb->s_umount);
-
-	ret2 = errseq_check_and_advance(&sb->s_wb_err, &f.file->f_sb_err);
+	if (f.file->f_op->syncfs)
+		ret = f.file->f_op->syncfs(f.file);
+	else
+		ret = generic_syncfs(f.file);
 
 	fdput(f);
-	return ret ? ret : ret2;
+	return ret;
 }
 
 /**
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 8667d0cdc71e..6710469b7e33 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1859,6 +1859,7 @@ struct file_operations {
 				   struct file *file_out, loff_t pos_out,
 				   loff_t len, unsigned int remap_flags);
 	int (*fadvise)(struct file *, loff_t, loff_t, int);
+	int (*syncfs)(struct file *);
 } __randomize_layout;
 
 struct inode_operations {
-- 
2.29.2