khugepaged: enable collapse pmd for pte-mapped THP

khugepaged needs exclusive mmap_sem to access page table.  When it fails
to lock mmap_sem, the page will fault in as pte-mapped THP.  As the page
is already a THP, khugepaged will not handle this pmd again.

This patch enables the khugepaged to retry collapse the page table.

struct mm_slot (in khugepaged.c) is extended with an array, containing
addresses of pte-mapped THPs.  We use array here for simplicity.  We can
easily replace it with more advanced data structures when needed.

In khugepaged_scan_mm_slot(), if the mm contains pte-mapped THP, we try to
collapse the page table.

Since collapse may happen at an later time, some pages may already fault
in.  collapse_pte_mapped_thp() is added to properly handle these pages.
collapse_pte_mapped_thp() also double checks whether all ptes in this pmd
are mapping to the same THP.  This is necessary because some subpage of
the THP may be replaced, for example by uprobe.  In such cases, it is not
possible to collapse the pmd.

[kirill.shutemov@linux.intel.com: add comments for retract_page_tables()]
  Link: http://lkml.kernel.org/r/20190816145443.6ard3iilytc6jlgv@box
Link: http://lkml.kernel.org/r/20190815164525.1848545-6-songliubraving@fb.com
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h
index 082d1d2a5216..bc45ea1efbf7 100644
--- a/include/linux/khugepaged.h
+++ b/include/linux/khugepaged.h
@@ -15,6 +15,14 @@ extern int __khugepaged_enter(struct mm_struct *mm);
 extern void __khugepaged_exit(struct mm_struct *mm);
 extern int khugepaged_enter_vma_merge(struct vm_area_struct *vma,
 				      unsigned long vm_flags);
+#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)
+{
+}
+#endif
 
 #define khugepaged_enabled()					       \
 	(transparent_hugepage_flags &				       \
@@ -73,6 +81,10 @@ static inline int khugepaged_enter_vma_merge(struct vm_area_struct *vma,
 {
 	return 0;
 }
+static inline void collapse_pte_mapped_thp(struct mm_struct *mm,
+					   unsigned long addr)
+{
+}
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 
 #endif /* _LINUX_KHUGEPAGED_H */