ext4: Optimization of no-op ext4_truncate triggers

v1: https://lore.kernel.org/lkml/[email protected]/T/

Changes from last version: Moved vfs-level changes to be ext4-level, and improved the description of the patch.

This patch enables skipping no-op 'ext4_truncate' calls. Analyzing the kernel with ftrace shows ext4_truncate is being sometimes called without making any impact, and sometimes userspace programs might call ext4_truncate in vein. By detecting these calls and skipping them, cpu time is saved.

I'll fix this by skipping ext4_truncate call in 'ext4_setattr' when the file's size hasn't changed AND it hasn't been truncated since the last disk space preallocation. It is meant to consider the case when ext4_truncate is being called to truncate preallocated blocks too. Notice that so far, the condition to triggering ext4_truncate by the user was: if (attr->ia_size <= oldsize) which means it is being triggered when attr->ia_size == oldsize regardless of whether there are preallocated blocks or not - if there are none, then the call is redundant.

Steps:
1.Add a new inode state flag: EXT4_STATE_TRUNCATED
2.Clear the flag when ext4_fallocate is being called with FALLOC_FL_KEEP_SIZE flag to enable using ext4_truncate again, to remove preallocated disk space that may have resulted from this call.
3.Set EXT4_STATE_TRUNCATED when ext4_truncated is called successfully. 4.Don't skip ext4_truncate in ext4_setattr when the size of the file has either been reduced OR stayed the same, but hasn't been truncated yet. This is in order to allow truncating of preallocated blocks.

---
 fs/ext4/ext4.h    | 1 +
 fs/ext4/extents.c | 4 ++++
 fs/ext4/inode.c   | 6 +++++-
 3 files changed, 10 insertions(+), 1 deletion(-)

index 44b0d418143c..34128a88e88f 100644
@@ -1915,6 +1915,7 @@ enum {
 	EXT4_STATE_VERITY_IN_PROGRESS,	/* building fs-verity Merkle tree */
 	EXT4_STATE_FC_COMMITTING,	/* Fast commit ongoing */
 	EXT4_STATE_ORPHAN_FILE,		/* Inode orphaned in orphan file */
+	EXT4_STATE_TRUNCATED,		/* Inode is truncated */
 };
 
 #define EXT4_INODE_BIT_FNS(name, field, offset)				\

index 34e25eee6521..b480c29dfc65 100644
@@ -4782,6 +4782,10 @@ long ext4_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
 		ret = ext4_zero_range(file, offset, len, mode);
 		goto exit;
 	}
+
+	if (mode & FALLOC_FL_KEEP_SIZE)
+		ext4_clear_inode_state(inode, EXT4_STATE_TRUNCATED);
+
 	trace_ext4_fallocate_enter(inode, offset, len, mode);
 	lblk = offset >> blkbits;
 

index 54bdd4884fe6..cbdad3253920 100644
@@ -4193,6 +4193,8 @@ int ext4_truncate(struct inode *inode)
 	if (IS_SYNC(inode))
 		ext4_handle_sync(handle);
 
+	ext4_set_inode_state(inode, EXT4_STATE_TRUNCATED);
+
 out_stop:
 	/*
 	 * If this was a simple ftruncate() and the file will remain alive,

@@ -5492,7 +5494,9 @@ int ext4_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
 		 * Call ext4_truncate() even if i_size didn't change to
 		 * truncate possible preallocated blocks.
 		 */
-		if (attr->ia_size <= oldsize) {
+		if (attr->ia_size < oldsize ||
+			(attr->ia_size == oldsize &&
+			!ext4_test_inode_state(inode, EXT4_STATE_TRUNCATED))) {
 			rc = ext4_truncate(inode);
 			if (rc)
 				error = rc;

2.43.0