Re: [PATCH v3 v3 5/7] ntfs3: add allocation tracepoints

--- Patch [5]: [PATCH v3 v3 5/7] ntfs3: add allocation tracepoints ---

commit 0592c540bc564c346cb524c624f6ee4a5768406b
Author: Baolin Liu [off-list ref]

ntfs3: add allocation tracepoints

This commit adds trace events for ntfs3 allocation operations. It introduces
events for attr_allocate_clusters, attr_set_size_ex, and attr_data_get_block
to help observe cluster allocation, size changes, and block mapping activity.

quoted

diff --git a/include/trace/events/ntfs3.h b/include/trace/events/ntfs3.h
index 4ac05ed29e49d..d16dd127ff2e5 100644
--- a/include/trace/events/ntfs3.h
+++ b/include/trace/events/ntfs3.h

[ ... ]

quoted

+TRACE_EVENT(ntfs3_attr_set_size_ex,
+	TP_PROTO(struct inode *inode, u32 type, u64 new_size, bool keep_prealloc,
+		 bool no_da),
+	TP_ARGS(inode, type, new_size, keep_prealloc, no_da),
+	TP_STRUCT__entry(
+		__field(loff_t, old_size)
+		__field(u64, new_size)
+		__field(unsigned long, ino)
+		__field(dev_t, dev)
+		__field(u32, type)
+		__field(bool, keep_prealloc)
+		__field(bool, no_da)
+	),
+	TP_fast_assign(
+		__entry->old_size = i_size_read(inode);

Will this actually record the previous size during truncate operations?

When an application truncates a file, the VFS size is often updated before 
calling into the filesystem. For instance, looking at how ntfs_setattr()
handles this:

fs/ntfs3/file.c:ntfs_setattr() {
        ...
        truncate_setsize(inode, newsize);
        
        err = ntfs_set_size(inode, newsize);
        ...
}

Since truncate_setsize() updates the VFS inode size before attr_set_size_ex()
and this tracepoint are reached, won't i_size_read(inode) just return the
newly truncated size, resulting in old_size equaling new_size?

Could the actual old size be passed as an explicit parameter to the 
tracepoint instead of sampling it here?

quoted

+		__entry->new_size = new_size;
+		__entry->ino = inode->i_ino;
+		__entry->dev = inode->i_sb->s_dev;
+		__entry->type = type;
+		__entry->keep_prealloc = keep_prealloc;
+		__entry->no_da = no_da;
+	),

[ ... ]