[Devel] [PATCH vz7] fuse: trust server file size unless opened

[Maxim Patlasov]

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>
---
 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 9cad8c5..62967d2 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);