mm/thp: unmap_mapping_page() to fix THP truncate_cleanup_page()

There is a race between THP unmapping and truncation, when truncate sees
pmd_none() and skips the entry, after munmap's zap_huge_pmd() cleared
it, but before its page_remove_rmap() gets to decrement
compound_mapcount: generating false "BUG: Bad page cache" reports that
the page is still mapped when deleted.  This commit fixes that, but not
in the way I hoped.

The first attempt used try_to_unmap(page, TTU_SYNC|TTU_IGNORE_MLOCK)
instead of unmap_mapping_range() in truncate_cleanup_page(): it has
often been an annoyance that we usually call unmap_mapping_range() with
no pages locked, but there apply it to a single locked page.
try_to_unmap() looks more suitable for a single locked page.

However, try_to_unmap_one() contains a VM_BUG_ON_PAGE(!pvmw.pte,page):
it is used to insert THP migration entries, but not used to unmap THPs.
Copy zap_huge_pmd() and add THP handling now? Perhaps, but their TLB
needs are different, I'm too ignorant of the DAX cases, and couldn't
decide how far to go for anon+swap.  Set that aside.

The second attempt took a different tack: make no change in truncate.c,
but modify zap_huge_pmd() to insert an invalidated huge pmd instead of
clearing it initially, then pmd_clear() between page_remove_rmap() and
unlocking at the end.  Nice.  But powerpc blows that approach out of the
water, with its serialize_against_pte_lookup(), and interesting pgtable
usage.  It would need serious help to get working on powerpc (with a
minor optimization issue on s390 too).  Set that aside.

Just add an "if (page_mapped(page)) synchronize_rcu();" or other such
delay, after unmapping in truncate_cleanup_page()? Perhaps, but though
that's likely to reduce or eliminate the number of incidents, it would
give less assurance of whether we had identified the problem correctly.

This successful iteration introduces "unmap_mapping_page(page)" instead
of try_to_unmap(), and goes the usual unmap_mapping_range_tree() route,
with an addition to details.  Then zap_pmd_range() watches for this
case, and does spin_unlock(pmd_lock) if so - just like
page_vma_mapped_walk() now does in the PVMW_SYNC case.  Not pretty, but
safe.

Note that unmap_mapping_page() is doing a VM_BUG_ON(!PageLocked) to
assert its interface; but currently that's only used to make sure that
page->mapping is stable, and zap_pmd_range() doesn't care if the page is
locked or not.  Along these lines, in invalidate_inode_pages2_range()
move the initial unmap_mapping_range() out from under page lock, before
then calling unmap_mapping_page() under page lock if still mapped.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: fc127da085c2 ("truncate: handle file thp")
Signed-off-by: Hugh Dickins <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Reviewed-by: Yang Shi <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Jue Wang <[email protected]>
Cc: "Matthew Wilcox (Oracle)" <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Ralph Campbell <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Wang Yugui <[email protected]>
Cc: Zi Yan <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 19 insertions, 24 deletions

diff --git a/mm/truncate.c b/mm/truncate.c
index 95af244b112a..234ddd879caa 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -167,13 +167,10 @@ void do_invalidatepage(struct page *page, unsigned int offset,
  * its lock, b) when a concurrent invalidate_mapping_pages got there first and
  * c) when tmpfs swizzles a page between a tmpfs inode and swapper_space.
  */
-static void
-truncate_cleanup_page(struct address_space *mapping, struct page *page)
+static void truncate_cleanup_page(struct page *page)
 {
-	if (page_mapped(page)) {
-		unsigned int nr = thp_nr_pages(page);
-		unmap_mapping_pages(mapping, page->index, nr, false);
-	}
+	if (page_mapped(page))
+		unmap_mapping_page(page);
 
 	if (page_has_private(page))
 		do_invalidatepage(page, 0, thp_size(page));
@@ -218,7 +215,7 @@ int truncate_inode_page(struct address_space *mapping, struct page *page)
 	if (page->mapping != mapping)
 		return -EIO;
 
-	truncate_cleanup_page(mapping, page);
+	truncate_cleanup_page(page);
 	delete_from_page_cache(page);
 	return 0;
 }
@@ -325,7 +322,7 @@ void truncate_inode_pages_range(struct address_space *mapping,
 		index = indices[pagevec_count(&pvec) - 1] + 1;
 		truncate_exceptional_pvec_entries(mapping, &pvec, indices);
 		for (i = 0; i < pagevec_count(&pvec); i++)
-			truncate_cleanup_page(mapping, pvec.pages[i]);
+			truncate_cleanup_page(pvec.pages[i]);
 		delete_from_page_cache_batch(mapping, &pvec);
 		for (i = 0; i < pagevec_count(&pvec); i++)
 			unlock_page(pvec.pages[i]);
@@ -639,6 +636,16 @@ int invalidate_inode_pages2_range(struct address_space *mapping,
 				continue;
 			}
 
+			if (!did_range_unmap && page_mapped(page)) {
+				/*
+				 * If page is mapped, before taking its lock,
+				 * zap the rest of the file in one hit.
+				 */
+				unmap_mapping_pages(mapping, index,
+						(1 + end - index), false);
+				did_range_unmap = 1;
+			}
+
 			lock_page(page);
 			WARN_ON(page_to_index(page) != index);
 			if (page->mapping != mapping) {
@@ -646,23 +653,11 @@ int invalidate_inode_pages2_range(struct address_space *mapping,
 				continue;
 			}
 			wait_on_page_writeback(page);
-			if (page_mapped(page)) {
-				if (!did_range_unmap) {
-					/*
-					 * Zap the rest of the file in one hit.
-					 */
-					unmap_mapping_pages(mapping, index,
-						(1 + end - index), false);
-					did_range_unmap = 1;
-				} else {
-					/*
-					 * Just zap this page
-					 */
-					unmap_mapping_pages(mapping, index,
-								1, false);
-				}
-			}
+
+			if (page_mapped(page))
+				unmap_mapping_page(page);
 			BUG_ON(page_mapped(page));
+
 			ret2 = do_launder_page(mapping, page);
 			if (ret2 == 0) {
 				if (!invalidate_complete_page2(mapping, page))