mm: khugepaged: recalculate min_free_kbytes after memory hotplug as expected by khugepaged

When memory is hotplug added or removed the min_free_kbytes should be
recalculated based on what is expected by khugepaged. Currently after
hotplug, min_free_kbytes will be set to a lower default and higher
default set when THP enabled is lost.

This change restores min_free_kbytes as expected for THP consumers.

[[e-mail]: v5]
Link: https://lkml.kernel.org/r/[e-mail]

Fixes: f000565adb77 ("thp: set recommended min free kbytes")
Signed-off-by: Vijay Balakrishna <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Reviewed-by: Pavel Tatashin <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Cc: Allen Pais <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: "Kirill A. Shutemov" <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: Song Liu <[e-mail]>
Cc: <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/khugepaged.h

···
 extern void __khugepaged_exit(struct mm_struct *mm);
 extern int khugepaged_enter_vma_merge(struct vm_area_struct *vma,
 				      unsigned long vm_flags);
+extern void khugepaged_min_free_kbytes_update(void);
 #ifdef CONFIG_SHMEM
 extern void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr);
 #else
···
 }
 static inline void collapse_pte_mapped_thp(struct mm_struct *mm,
 					   unsigned long addr)
+{
+}
+
+static inline void khugepaged_min_free_kbytes_update(void)
 {
 }
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */

mm/khugepaged.c

···
 #define CREATE_TRACE_POINTS
 #include <trace/events/huge_memory.h>
 
+static struct task_struct *khugepaged_thread __read_mostly;
+static DEFINE_MUTEX(khugepaged_mutex);
+
 /* default scan 8*512 pte (or vmas) every 30 second */
 static unsigned int khugepaged_pages_to_scan __read_mostly;
 static unsigned int khugepaged_pages_collapsed;
···
 
 int start_stop_khugepaged(void)
 {
-	static struct task_struct *khugepaged_thread __read_mostly;
-	static DEFINE_MUTEX(khugepaged_mutex);
 	int err = 0;
 
 	mutex_lock(&khugepaged_mutex);
···
 fail:
 	mutex_unlock(&khugepaged_mutex);
 	return err;
+}
+
+void khugepaged_min_free_kbytes_update(void)
+{
+	mutex_lock(&khugepaged_mutex);
+	if (khugepaged_enabled() && khugepaged_thread)
+		set_recommended_min_free_kbytes();
+	mutex_unlock(&khugepaged_mutex);
 }

mm/page_alloc.c

···
 #include <linux/nmi.h>
 #include <linux/psi.h>
 #include <linux/padata.h>
+#include <linux/khugepaged.h>
 
 #include <asm/sections.h>
 #include <asm/tlbflush.h>
···
 	setup_min_unmapped_ratio();
 	setup_min_slab_ratio();
 #endif
+
+	khugepaged_min_free_kbytes_update();
 
 	return 0;
 }