diff mbox series

[018/156] mm/filemap: fix storing to a THP shadow entry

Message ID 20201016024159.MuK94mohm%akpm@linux-foundation.org
State New
Headers show
Series [001/156] device-dax/kmem: fix resource release | expand

Commit Message

Andrew Morton Oct. 16, 2020, 2:41 a.m. UTC
From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Subject: mm/filemap: fix storing to a THP shadow entry

When a THP is removed from the page cache by reclaim, we replace it with a
shadow entry that occupies all slots of the XArray previously occupied by
the THP.  If the user then accesses that page again, we only allocate a
single page, but storing it into the shadow entry replaces all entries
with that one page.  That leads to bugs like

page dumped because: VM_BUG_ON_PAGE(page_to_pgoff(page) != offset)
------------[ cut here ]------------
kernel BUG at mm/filemap.c:2529!

https://bugzilla.kernel.org/show_bug.cgi?id=206569

This is hard to reproduce with mainline, but happens regularly with the
THP patchset (as so many more THPs are created).  This solution is take
from the THP patchset.  It splits the shadow entry into order-0 pieces at
the time that we bring a new page into cache.

Link: https://lkml.kernel.org/r/20200903183029.14930-4-willy@infradead.org
Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Qian Cai <cai@lca.pw>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/filemap.c |   42 +++++++++++++++++++++++++++++++-----------
 1 file changed, 31 insertions(+), 11 deletions(-)
diff mbox series

Patch

--- a/mm/filemap.c~mm-filemap-fix-storing-to-a-thp-shadow-entry
+++ a/mm/filemap.c
@@ -829,13 +829,12 @@  EXPORT_SYMBOL_GPL(replace_page_cache_pag
 
 static int __add_to_page_cache_locked(struct page *page,
 				      struct address_space *mapping,
-				      pgoff_t offset, gfp_t gfp_mask,
+				      pgoff_t offset, gfp_t gfp,
 				      void **shadowp)
 {
 	XA_STATE(xas, &mapping->i_pages, offset);
 	int huge = PageHuge(page);
 	int error;
-	void *old;
 
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
 	VM_BUG_ON_PAGE(PageSwapBacked(page), page);
@@ -846,25 +845,46 @@  static int __add_to_page_cache_locked(st
 	page->index = offset;
 
 	if (!huge) {
-		error = mem_cgroup_charge(page, current->mm, gfp_mask);
+		error = mem_cgroup_charge(page, current->mm, gfp);
 		if (error)
 			goto error;
 	}
 
+	gfp &= GFP_RECLAIM_MASK;
+
 	do {
+		unsigned int order = xa_get_order(xas.xa, xas.xa_index);
+		void *entry, *old = NULL;
+
+		if (order > thp_order(page))
+			xas_split_alloc(&xas, xa_load(xas.xa, xas.xa_index),
+					order, gfp);
 		xas_lock_irq(&xas);
-		old = xas_load(&xas);
-		if (old && !xa_is_value(old))
-			xas_set_err(&xas, -EEXIST);
+		xas_for_each_conflict(&xas, entry) {
+			old = entry;
+			if (!xa_is_value(entry)) {
+				xas_set_err(&xas, -EEXIST);
+				goto unlock;
+			}
+		}
+
+		if (old) {
+			if (shadowp)
+				*shadowp = old;
+			/* entry may have been split before we acquired lock */
+			order = xa_get_order(xas.xa, xas.xa_index);
+			if (order > thp_order(page)) {
+				xas_split(&xas, old, order);
+				xas_reset(&xas);
+			}
+		}
+
 		xas_store(&xas, page);
 		if (xas_error(&xas))
 			goto unlock;
 
-		if (xa_is_value(old)) {
+		if (old)
 			mapping->nrexceptional--;
-			if (shadowp)
-				*shadowp = old;
-		}
 		mapping->nrpages++;
 
 		/* hugetlb pages do not participate in page cache accounting */
@@ -872,7 +892,7 @@  static int __add_to_page_cache_locked(st
 			__inc_lruvec_page_state(page, NR_FILE_PAGES);
 unlock:
 		xas_unlock_irq(&xas);
-	} while (xas_nomem(&xas, gfp_mask & GFP_RECLAIM_MASK));
+	} while (xas_nomem(&xas, gfp));
 
 	if (xas_error(&xas)) {
 		error = xas_error(&xas);