[Devel] [PATCH RHEL7 COMMIT] fuse: trust server file size unless opened

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-327.36.1.vz7.20.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-327.36.1.vz7.20.15
------>
commit b6b7fc7f1d2dbfef8fb8a8e109e33cb770ce43ac
Author: Maxim Patlasov <mpatlasov at virtuozzo.com>
Date:   Thu Dec 15 17:48:29 2016 +0400

    fuse: trust server file size unless opened
    
    Before the patch, the only way to pick up updated file size from server (in a
    scenario when local inode was created earlier, then the file was updated
    from another node) was in fuse_open_common():
    
    > 		atomic_inc(&fi->num_openers);
    >
    > 		if (atomic_read(&fi->num_openers) == 1) {
    > 			err = fuse_getattr_size(inode, file, &size);
    > 			...
    > 			spin_lock(&fc->lock);
    > 			i_size_write(inode, size);
    > 			spin_unlock(&fc->lock);
    > 		}
    
    This is correct, but someone may ask about i_size w/o open, e.g.: ls -l foo.
    The patch ensures that every time the server reports us some file size, if no
    open-s happened yet (num_openers=0), fuse stores that server size in local
    inode->i_size. This resolves the following problem:
    
    # pstorage-mount -c test -l /var/log/f1.log /pcs1
    # pstorage-mount -c test -l /var/log/f2.log /pcs2
    
    # date > /pcs1/foo; ls -l /pcs1/foo /pcs2/foo
    -rwx------ 1 root root 29 Dec 14 16:31 /pcs1/foo
    -rwx------ 1 root root 29 Dec 14 16:31 /pcs2/foo
    
    # date >> /pcs1/foo; ls -l /pcs1/foo /pcs2/foo
    -rwx------ 1 root root 58 Dec 14 16:31 /pcs1/foo
    -rwx------ 1 root root 29 Dec 14 16:31 /pcs2/foo
    
    https://jira.sw.ru/browse/PSBM-57047
    
    Signed-off-by: Maxim Patlasov <mpatlasov at virtuozzo.com>
    Acked-by: Dmitry Monakhov <dmonakhov at openvz.org>
---
 fs/fuse/file.c   | 12 +++++++++++-
 fs/fuse/fuse_i.h |  3 +++
 fs/fuse/inode.c  |  4 +++-
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 501aa27..1bda994 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -296,12 +296,20 @@ int fuse_open_common(struct inode *inode, struct file *file, bool isdir)
 		u64 size;
 
 		mutex_lock(&inode->i_mutex);
+
+		spin_lock(&fc->lock);
 		atomic_inc(&fi->num_openers);
 
 		if (atomic_read(&fi->num_openers) == 1) {
+			fi->i_size_unstable = 1;
+			spin_unlock(&fc->lock);
 			err = fuse_getattr_size(inode, file, &size);
 			if (err) {
+				spin_lock(&fc->lock);
 				atomic_dec(&fi->num_openers);
+				fi->i_size_unstable = 0;
+				spin_unlock(&fc->lock);
+
 				mutex_unlock(&inode->i_mutex);
 				fuse_release_common(file, FUSE_RELEASE);
 				return err;
@@ -309,8 +317,10 @@ int fuse_open_common(struct inode *inode, struct file *file, bool isdir)
 
 			spin_lock(&fc->lock);
 			i_size_write(inode, size);
+			fi->i_size_unstable = 0;
+			spin_unlock(&fc->lock);
+		} else
 			spin_unlock(&fc->lock);
-		}
 
 		mutex_unlock(&inode->i_mutex);
 	}
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 1d24bf6..22eb9c9 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -124,6 +124,9 @@ struct fuse_inode {
 
 	/** Mostly to detect very first open */
 	atomic_t num_openers;
+
+	/** Even though num_openers>0, trust server i_size */
+	int i_size_unstable;
 };
 
 /** FUSE inode state bits */
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 5ccecae..f606deb 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -97,6 +97,7 @@ static struct inode *fuse_alloc_inode(struct super_block *sb)
 	fi->writectr = 0;
 	fi->orig_ino = 0;
 	fi->state = 0;
+	fi->i_size_unstable = 0;
 	INIT_LIST_HEAD(&fi->write_files);
 	INIT_LIST_HEAD(&fi->rw_files);
 	INIT_LIST_HEAD(&fi->queued_writes);
@@ -226,7 +227,8 @@ void fuse_change_attributes(struct inode *inode, struct fuse_attr *attr,
 	 * extend local i_size without keeping userspace server in sync. So,
 	 * attr->size coming from server can be stale. We cannot trust it.
 	 */
-	if (!is_wb || !S_ISREG(inode->i_mode))
+	if (!is_wb || !S_ISREG(inode->i_mode) ||
+	    !atomic_read(&fi->num_openers) || fi->i_size_unstable)
 		i_size_write(inode, attr->size);
 	spin_unlock(&fc->lock);