mm: accelerate munlock() treatment of THP pages

munlock_vma_pages_range() was always incrementing addresses by PAGE_SIZE
at a time.  When munlocking THP pages (or the huge zero page), this
resulted in taking the mm->page_table_lock 512 times in a row.

We can do better by making use of the page_mask returned by
follow_page_mask (for the huge zero page case), or the size of the page
munlock_vma_page() operated on (for the true THP page case).

Signed-off-by: Michel Lespinasse <walken@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 24 insertions, 12 deletions

diff --git a/mm/internal.h b/mm/internal.h
index 1c0c4cc0fcf7..8562de0a5197 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -195,7 +195,7 @@ static inline int mlocked_vma_newpage(struct vm_area_struct *vma,
  * must be called with vma's mmap_sem held for read or write, and page locked.
  */
 extern void mlock_vma_page(struct page *page);
-extern void munlock_vma_page(struct page *page);
+extern unsigned int munlock_vma_page(struct page *page);
 
 /*
  * Clear the page's PageMlocked().  This can be useful in a situation where
diff --git a/mm/mlock.c b/mm/mlock.c
index e6638f565d42..1c5e33fce639 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -102,13 +102,16 @@ void mlock_vma_page(struct page *page)
  * can't isolate the page, we leave it for putback_lru_page() and vmscan
  * [page_referenced()/try_to_unmap()] to deal with.
  */
-void munlock_vma_page(struct page *page)
+unsigned int munlock_vma_page(struct page *page)
 {
+        unsigned int page_mask = 0;
+
         BUG_ON(!PageLocked(page));
 
         if (TestClearPageMlocked(page)) {
-                mod_zone_page_state(page_zone(page), NR_MLOCK,
-                                    -hpage_nr_pages(page));
+                unsigned int nr_pages = hpage_nr_pages(page);
+                mod_zone_page_state(page_zone(page), NR_MLOCK, -nr_pages);
+                page_mask = nr_pages - 1;
                 if (!isolate_lru_page(page)) {
                         int ret = SWAP_AGAIN;
 
@@ -141,6 +144,8 @@ void munlock_vma_page(struct page *page)
                                 count_vm_event(UNEVICTABLE_PGMUNLOCKED);
                 }
         }
+
+        return page_mask;
 }
 
 /**
@@ -159,7 +164,6 @@ long __mlock_vma_pages_range(struct vm_area_struct *vma,
                 unsigned long start, unsigned long end, int *nonblocking)
 {
         struct mm_struct *mm = vma->vm_mm;
-        unsigned long addr = start;
         unsigned long nr_pages = (end - start) / PAGE_SIZE;
         int gup_flags;
 
@@ -189,7 +193,7 @@ long __mlock_vma_pages_range(struct vm_area_struct *vma,
          * We made sure addr is within a VMA, so the following will
          * not result in a stack expansion that recurses back here.
          */
-        return __get_user_pages(current, mm, addr, nr_pages, gup_flags,
+        return __get_user_pages(current, mm, start, nr_pages, gup_flags,
                                 NULL, NULL, nonblocking);
 }
 
@@ -226,13 +230,12 @@ static int __mlock_posix_error_return(long retval)
 void munlock_vma_pages_range(struct vm_area_struct *vma,
                              unsigned long start, unsigned long end)
 {
-        unsigned long addr;
-
-        lru_add_drain();
         vma->vm_flags &= ~VM_LOCKED;
 
-        for (addr = start; addr < end; addr += PAGE_SIZE) {
+        while (start < end) {
                 struct page *page;
+                unsigned int page_mask, page_increm;
+
                 /*
                  * Although FOLL_DUMP is intended for get_dump_page(),
                  * it just so happens that its special treatment of the
@@ -240,13 +243,22 @@ void munlock_vma_pages_range(struct vm_area_struct *vma,
                  * suits munlock very well (and if somehow an abnormal page
                  * has sneaked into the range, we won't oops here: great).
                  */
-                page = follow_page(vma, addr, FOLL_GET | FOLL_DUMP);
+                page = follow_page_mask(vma, start, FOLL_GET | FOLL_DUMP,
+                                        &page_mask);
                 if (page && !IS_ERR(page)) {
                         lock_page(page);
-                        munlock_vma_page(page);
+                        lru_add_drain();
+                        /*
+                         * Any THP page found by follow_page_mask() may have
+                         * gotten split before reaching munlock_vma_page(),
+                         * so we need to recompute the page_mask here.
+                         */
+                        page_mask = munlock_vma_page(page);
                         unlock_page(page);
                         put_page(page);
                 }
+                page_increm = 1 + (~(start >> PAGE_SHIFT) & page_mask);
+                start += page_increm * PAGE_SIZE;
                 cond_resched();
         }
 }