xfs: restore speculative_cow_prealloc_lifetime sysctl

In commit 9669f51de5c0 I tried to get rid of the undocumented cow gc
lifetime knob. The knob's function was never documented and it now
doesn't really have a function since eof and cow gc have been
consolidated.

Regrettably, xfs/231 relies on it and regresses on for-next. I did not
succeed at getting far enough through fstests patch review for the fixup
to land in time.

Restore the sysctl knob, document what it did (does?), put it on the
deprecation schedule, and rip out a redundant function.

Fixes: 9669f51de5c0 ("xfs: consolidate the eofblocks and cowblocks workers")
Signed-off-by: Darrick J. Wong <[e-mail]>
Reviewed-by: Dave Chinner <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>

Documentation/admin-guide/xfs.rst

···
 	removes unused preallocation from clean inodes and releases
 	the unused space back to the free pool.
 
+  fs.xfs.speculative_cow_prealloc_lifetime
+	This is an alias for speculative_prealloc_lifetime.
+
   fs.xfs.error_level		(Min: 0  Default: 3  Max: 11)
 	A volume knob for error reporting when internal errors occur.
 	This will generate detailed messages & backtraces for filesystem
···
 Deprecated Sysctls
 ==================
 
-===========================     ================
-  Name				Removal Schedule
-===========================     ================
-fs.xfs.irix_sgid_inherit        September 2025
-fs.xfs.irix_symlink_mode        September 2025
-===========================     ================
+===========================================     ================
+  Name                                          Removal Schedule
+===========================================     ================
+fs.xfs.irix_sgid_inherit                        September 2025
+fs.xfs.irix_symlink_mode                        September 2025
+fs.xfs.speculative_cow_prealloc_lifetime        September 2025
+===========================================     ================
 
 
 Removed Sysctls

fs/xfs/xfs_sysctl.c

···
 #endif /* CONFIG_PROC_FS */
 
 STATIC int
-xfs_deprecate_irix_sgid_inherit_proc_handler(
+xfs_deprecated_dointvec_minmax(
 	struct ctl_table	*ctl,
 	int			write,
 	void			*buffer,
···
 	loff_t			*ppos)
 {
 	if (write) {
-		printk_once(KERN_WARNING
-				"XFS: " "%s sysctl option is deprecated.\n",
-				ctl->procname);
-	}
-	return proc_dointvec_minmax(ctl, write, buffer, lenp, ppos);
-}
-
-STATIC int
-xfs_deprecate_irix_symlink_mode_proc_handler(
-	struct ctl_table	*ctl,
-	int			write,
-	void			*buffer,
-	size_t			*lenp,
-	loff_t			*ppos)
-{
-	if (write) {
-		printk_once(KERN_WARNING
-				"XFS: " "%s sysctl option is deprecated.\n",
+		printk_ratelimited(KERN_WARNING
+				"XFS: %s sysctl option is deprecated.\n",
 				ctl->procname);
 	}
 	return proc_dointvec_minmax(ctl, write, buffer, lenp, ppos);
···
 		.data		= &xfs_params.sgid_inherit.val,
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
-		.proc_handler	= xfs_deprecate_irix_sgid_inherit_proc_handler,
+		.proc_handler	= xfs_deprecated_dointvec_minmax,
 		.extra1		= &xfs_params.sgid_inherit.min,
 		.extra2		= &xfs_params.sgid_inherit.max
 	},
···
 		.data		= &xfs_params.symlink_mode.val,
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
-		.proc_handler	= xfs_deprecate_irix_symlink_mode_proc_handler,
+		.proc_handler	= xfs_deprecated_dointvec_minmax,
 		.extra1		= &xfs_params.symlink_mode.min,
 		.extra2		= &xfs_params.symlink_mode.max
 	},
···
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= &xfs_params.blockgc_timer.min,
+		.extra2		= &xfs_params.blockgc_timer.max,
+	},
+	{
+		.procname	= "speculative_cow_prealloc_lifetime",
+		.data		= &xfs_params.blockgc_timer.val,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= xfs_deprecated_dointvec_minmax,
 		.extra1		= &xfs_params.blockgc_timer.min,
 		.extra2		= &xfs_params.blockgc_timer.max,
 	},