[PATCH 09/19] mm/filemap: Allow THPs to be added to the page cache

["Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>]

We return -EEXIST if there are any non-shadow entries in the page
cache in the range covered by the THP.  If there are multiple
shadow entries in the range, we set *shadowp to one of them (currently
the one at the highest index).  If that turns out to be the wrong
answer, we can implement something more complex.  This is mostly
modelled after the equivalent function in the shmem code.

Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 mm/filemap.c | 37 ++++++++++++++++++++++---------------
 1 file changed, 22 insertions(+), 15 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 64fe0018ee17..dabc26cf0067 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -811,23 +811,25 @@ noinline int __add_to_page_cache_locked(struct page *page,
 {
 	XA_STATE(xas, &mapping->i_pages, offset);
 	int huge = PageHuge(page);
-	int error;
+	unsigned int nr = 1;
 
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
 	VM_BUG_ON_PAGE(PageSwapBacked(page), page);
 	mapping_set_update(&xas, mapping);
 
-	get_page(page);
-	page->mapping = mapping;
-	page->index = offset;
-
 	if (!huge) {
-		error = mem_cgroup_charge(page, current->mm, gfp);
+		int error = mem_cgroup_charge(page, current->mm, gfp);
+
 		if (error)
-			goto error;
+			return error;
+		xas_set_order(&xas, offset, thp_order(page));
+		nr = thp_nr_pages(page);
 	}
 
 	gfp &= GFP_RECLAIM_MASK;
+	page_ref_add(page, nr);
+	page->mapping = mapping;
+	page->index = xas.xa_index;
 
 	do {
 		unsigned int order = xa_get_order(xas.xa, xas.xa_index);
@@ -851,6 +853,8 @@ noinline int __add_to_page_cache_locked(struct page *page,
 			/* entry may have been split before we acquired lock */
 			order = xa_get_order(xas.xa, xas.xa_index);
 			if (order > thp_order(page)) {
+				/* How to handle large swap entries? */
+				BUG_ON(shmem_mapping(mapping));
 				xas_split(&xas, old, order);
 				xas_reset(&xas);
 			}
@@ -860,27 +864,30 @@ noinline int __add_to_page_cache_locked(struct page *page,
 		if (xas_error(&xas))
 			goto unlock;
 
-		mapping->nrpages++;
+		mapping->nrpages += nr;
 
 		/* hugetlb pages do not participate in page cache accounting */
-		if (!huge)
-			__inc_lruvec_page_state(page, NR_FILE_PAGES);
+		if (!huge) {
+			__mod_lruvec_page_state(page, NR_FILE_PAGES, nr);
+			if (nr > 1)
+				__mod_node_page_state(page_pgdat(page),
+						NR_FILE_THPS, nr);
+		}
 unlock:
 		xas_unlock_irq(&xas);
 	} while (xas_nomem(&xas, gfp));
 
-	if (xas_error(&xas)) {
-		error = xas_error(&xas);
+	if (xas_error(&xas))
 		goto error;
-	}
 
 	trace_mm_filemap_add_to_page_cache(page);
 	return 0;
 error:
 	page->mapping = NULL;
 	/* Leave page->index set: truncation relies upon it */
-	put_page(page);
-	return error;
+	page_ref_sub(page, nr);
+	VM_BUG_ON_PAGE(page_count(page) <= 0, page);
+	return xas_error(&xas);
 }
 ALLOW_ERROR_INJECTION(__add_to_page_cache_locked, ERRNO);
 
-- 
2.28.0