Commit 6923aa0d authored by Sebastian Andrzej Siewior's avatar Sebastian Andrzej Siewior Committed by Linus Torvalds
Browse files

mm/compaction: Disable compact_unevictable_allowed on RT 

Since commit 5bbe3547 ("mm: allow compaction of unevictable pages")
it is allowed to examine mlocked pages and compact them by default.  On
-RT even minor pagefaults are problematic because it may take a few 100us
to resolve them and until then the task is blocked.

Make compact_unevictable_allowed = 0 default and issue a warning on RT if
it is changed.

[bigeasy@linutronix.de: v5]
  Link: https://lore.kernel.org/linux-mm/20190710144138.qyn4tuttdq6h7kqx@linutronix.de/
  Link: http://lkml.kernel.org/r/20200319165536.ovi75tsr2seared4@linutronix.de


Signed-off-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Reviewed-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Acked-by: default avatarMel Gorman <mgorman@techsingularity.net>
Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Iurii Zaikin <yzaikin@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Link: https://lore.kernel.org/linux-mm/20190710144138.qyn4tuttdq6h7kqx@linutronix.de/
Link: http://lkml.kernel.org/r/20200303202225.nhqc3v5gwlb7x6et@linutronix.de


Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 964b692d

Documentation/admin-guide/sysctl/vm.rst

+3 −0
Original line number Diff line number Diff line
@@ -128,6 +128,9 @@ allowed to examine the unevictable lru (mlocked pages) for pages to compact. 
This should be used on systems where stalls for minor page faults are an

acceptable trade for large contiguous free memory.  Set to 0 to prevent

compaction from moving pages that are unevictable.  Default value is 1.

On CONFIG_PREEMPT_RT the default value is 0 in order to avoid a page fault, due

to compaction, which would block the task from becomming active until the fault

is resolved.





dirty_background_bytes

kernel/sysctl.c

+28 −1
Original line number Diff line number Diff line
@@ -212,6 +212,11 @@ static int proc_do_cad_pid(struct ctl_table *table, int write, 
		  void __user *buffer, size_t *lenp, loff_t *ppos);

static int proc_taint(struct ctl_table *table, int write,

			       void __user *buffer, size_t *lenp, loff_t *ppos);

#ifdef CONFIG_COMPACTION

static int proc_dointvec_minmax_warn_RT_change(struct ctl_table *table,

					       int write, void __user *buffer,

					       size_t *lenp, loff_t *ppos);

#endif

#endif



#ifdef CONFIG_PRINTK
@@ -1467,7 +1472,7 @@ static struct ctl_table vm_table[] = { 
		.data		= &sysctl_compact_unevictable_allowed,

		.maxlen		= sizeof(int),

		.mode		= 0644,

		.proc_handler	= proc_dointvec_minmax,

		.proc_handler	= proc_dointvec_minmax_warn_RT_change,

		.extra1		= SYSCTL_ZERO,

		.extra2		= SYSCTL_ONE,

	},
@@ -2555,6 +2560,28 @@ int proc_dointvec(struct ctl_table *table, int write, 
	return do_proc_dointvec(table, write, buffer, lenp, ppos, NULL, NULL);

}



#ifdef CONFIG_COMPACTION

static int proc_dointvec_minmax_warn_RT_change(struct ctl_table *table,

					       int write, void __user *buffer,

					       size_t *lenp, loff_t *ppos)

{

	int ret, old;



	if (!IS_ENABLED(CONFIG_PREEMPT_RT) || !write)

		return proc_dointvec_minmax(table, write, buffer, lenp, ppos);



	old = *(int *)table->data;

	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);

	if (ret)

		return ret;

	if (old != *(int *)table->data)

		pr_warn_once("sysctl attribute %s changed by %s[%d]\n",

			     table->procname, current->comm,

			     task_pid_nr(current));

	return ret;

}

#endif



/**

 * proc_douintvec - read a vector of unsigned integers

 * @table: the sysctl table

mm/compaction.c

+4 −0
Original line number Diff line number Diff line
@@ -1594,7 +1594,11 @@ typedef enum { 
 * Allow userspace to control policy on scanning the unevictable LRU for

 * compactable pages.

 */

#ifdef CONFIG_PREEMPT_RT

int sysctl_compact_unevictable_allowed __read_mostly = 0;

#else

int sysctl_compact_unevictable_allowed __read_mostly = 1;

#endif



static inline void

update_fast_start_pfn(struct compact_control *cc, unsigned long pfn)

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