[PATCH] shmem: recalculate file inode when fstat

[Hugh Dickins]

From: Yu Zhao <yuzhao@xxxxxxxxxx>

Shmem uses shmem_recalc_inode to update i_blocks when it allocates
page, undoes range or swaps. But mm can drop clean page without
notifying shmem. This makes fstat sometimes return out-of-date
block size.

The problem can be partially solved when we add
inode_operations->getattr which calls shmem_recalc_inode to update
i_blocks for fstat.

shmem_recalc_inode also updates counter used by statfs and
vm_committed_as. For them the situation is not changed. They still
suffer from the discrepancy after dropping clean page and before
the function is called by aforementioned triggers.

Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
 mm/shmem.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index 4caf8ed..37e7933 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -542,6 +542,21 @@ void shmem_truncate_range(struct inode *inode, loff_t lstart, loff_t lend)
 }
 EXPORT_SYMBOL_GPL(shmem_truncate_range);
 
+static int shmem_getattr(struct vfsmount *mnt, struct dentry *dentry,
+			 struct kstat *stat)
+{
+	struct inode *inode = dentry->d_inode;
+	struct shmem_inode_info *info = SHMEM_I(inode);
+
+	spin_lock(&info->lock);
+	shmem_recalc_inode(inode);
+	spin_unlock(&info->lock);
+
+	generic_fillattr(inode, stat);
+
+	return 0;
+}
+
 static int shmem_setattr(struct dentry *dentry, struct iattr *attr)
 {
 	struct inode *inode = d_inode(dentry);
@@ -3122,6 +3137,7 @@ static const struct file_operations shmem_file_operations = {
 };
 
 static const struct inode_operations shmem_inode_operations = {
+	.getattr	= shmem_getattr,
 	.setattr	= shmem_setattr,
 #ifdef CONFIG_TMPFS_XATTR
 	.setxattr	= shmem_setxattr,
-- 
2.4.3.573.g4eafbef
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