thp: allow mlocked THP again

Before THP refcounting rework, THP was not allowed to cross VMA
boundary.  So, if we have THP and we split it, PG_mlocked can be safely
transferred to small pages.

With new THP refcounting and naive approach to mlocking we can end up
with this scenario:
 1. we have a mlocked THP, which belong to one VM_LOCKED VMA.
 2. the process does munlock() on the *part* of the THP:
      - the VMA is split into two, one of them VM_LOCKED;
      - huge PMD split into PTE table;
      - THP is still mlocked;
 3. split_huge_page():
      - it transfers PG_mlocked to *all* small pages regrardless if it
	blong to any VM_LOCKED VMA.

We probably could munlock() all small pages on split_huge_page(), but I
think we have accounting issue already on step two.

Instead of forbidding mlocked pages altogether, we just avoid mlocking
PTE-mapped THPs and munlock THPs on split_huge_pmd().

This means PTE-mapped THPs will be on normal lru lists and will be split
under memory pressure by vmscan.  After the split vmscan will detect
unevictable small pages and mlock them.

With this approach we shouldn't hit situation like described above.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Cc: Jerome Marchand <jmarchan@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Steve Capper <steve.capper@linaro.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 47 insertions, 21 deletions

diff --git a/mm/mlock.c b/mm/mlock.c
index c6b139ad356a..9197b6721a1e 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -82,6 +82,9 @@ void mlock_vma_page(struct page *page)
 	/* Serialize with page migration */
 	BUG_ON(!PageLocked(page));
 
+	VM_BUG_ON_PAGE(PageTail(page), page);
+	VM_BUG_ON_PAGE(PageCompound(page) && PageDoubleMap(page), page);
+
 	if (!TestSetPageMlocked(page)) {
 		mod_zone_page_state(page_zone(page), NR_MLOCK,
 				    hpage_nr_pages(page));
@@ -178,6 +181,8 @@ unsigned int munlock_vma_page(struct page *page)
 	/* For try_to_munlock() and to serialize with page migration */
 	BUG_ON(!PageLocked(page));
 
+	VM_BUG_ON_PAGE(PageTail(page), page);
+
 	/*
 	 * Serialize with any parallel __split_huge_page_refcount() which
 	 * might otherwise copy PageMlocked to part of the tail pages before
@@ -388,6 +393,13 @@ static unsigned long __munlock_pagevec_fill(struct pagevec *pvec,
 		if (!page || page_zone_id(page) != zoneid)
 			break;
 
+		/*
+		 * Do not use pagevec for PTE-mapped THP,
+		 * munlock_vma_pages_range() will handle them.
+		 */
+		if (PageTransCompound(page))
+			break;
+
 		get_page(page);
 		/*
 		 * Increase the address that will be returned *before* the
@@ -443,29 +455,43 @@ void munlock_vma_pages_range(struct vm_area_struct *vma,
 		page = follow_page_mask(vma, start, FOLL_GET | FOLL_DUMP,
 				&page_mask);
 
-		if (page && !IS_ERR(page) && !PageTransCompound(page)) {
-			/*
-			 * Non-huge pages are handled in batches via
-			 * pagevec. The pin from follow_page_mask()
-			 * prevents them from collapsing by THP.
-			 */
-			pagevec_add(&pvec, page);
-			zone = page_zone(page);
-			zoneid = page_zone_id(page);
+		if (page && !IS_ERR(page)) {
+			if (PageTransTail(page)) {
+				VM_BUG_ON_PAGE(PageMlocked(page), page);
+				put_page(page); /* follow_page_mask() */
+			} else if (PageTransHuge(page)) {
+				lock_page(page);
+				/*
+				 * 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); /* follow_page_mask() */
+			} else {
+				/*
+				 * Non-huge pages are handled in batches via
+				 * pagevec. The pin from follow_page_mask()
+				 * prevents them from collapsing by THP.
+				 */
+				pagevec_add(&pvec, page);
+				zone = page_zone(page);
+				zoneid = page_zone_id(page);
 
-			/*
-			 * Try to fill the rest of pagevec using fast
-			 * pte walk. This will also update start to
-			 * the next page to process. Then munlock the
-			 * pagevec.
-			 */
-			start = __munlock_pagevec_fill(&pvec, vma,
-					zoneid, start, end);
-			__munlock_pagevec(&pvec, zone);
-			goto next;
+				/*
+				 * Try to fill the rest of pagevec using fast
+				 * pte walk. This will also update start to
+				 * the next page to process. Then munlock the
+				 * pagevec.
+				 */
+				start = __munlock_pagevec_fill(&pvec, vma,
+						zoneid, start, end);
+				__munlock_pagevec(&pvec, zone);
+				goto next;
+			}
 		}
-		/* It's a bug to munlock in the middle of a THP page */
-		VM_BUG_ON((start >> PAGE_SHIFT) & page_mask);
 		page_increm = 1 + page_mask;
 		start += page_increm * PAGE_SIZE;
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