[PATCH v6 35/51] mm: Allow THPs to be added to the page cache

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

From: "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 | 52 +++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 35 insertions(+), 17 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 3a9579a1ffa7..ab9746aff766 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -834,41 +834,58 @@ static 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;
 	void *old;
 
 	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_mask);
 		if (error)
-			goto error;
+			return error;
+		xas_set_order(&xas, offset, thp_order(page));
+		nr = thp_nr_pages(page);
 	}
 
+	page_ref_add(page, nr);
+	page->mapping = mapping;
+	page->index = offset;
+
 	do {
+		unsigned long exceptional = 0;
+		unsigned int i = 0;
+
 		xas_lock_irq(&xas);
-		old = xas_load(&xas);
-		if (old && !xa_is_value(old))
-			xas_set_err(&xas, -EEXIST);
-		xas_store(&xas, page);
+		xas_for_each_conflict(&xas, old) {
+			if (!xa_is_value(old)) {
+				xas_set_err(&xas, -EEXIST);
+				break;
+			}
+			exceptional++;
+			if (shadowp)
+				*shadowp = old;
+		}
+		xas_create_range(&xas);
 		if (xas_error(&xas))
 			goto unlock;
 
-		if (xa_is_value(old)) {
-			mapping->nrexceptional--;
-			if (shadowp)
-				*shadowp = old;
+next:
+		xas_store(&xas, page);
+		if (++i < nr) {
+			xas_next(&xas);
+			goto next;
 		}
-		mapping->nrpages++;
+		mapping->nrexceptional -= exceptional;
+		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)
+				__inc_lruvec_page_state(page, NR_FILE_THPS);
+		}
 unlock:
 		xas_unlock_irq(&xas);
 	} while (xas_nomem(&xas, gfp_mask & GFP_RECLAIM_MASK));
@@ -883,7 +900,8 @@ static int __add_to_page_cache_locked(struct page *page,
 error:
 	page->mapping = NULL;
 	/* Leave page->index set: truncation relies upon it */
-	put_page(page);
+	page_ref_sub(page, nr);
+	VM_BUG_ON_PAGE(page_count(page) <= 0, page);
 	return error;
 }
 ALLOW_ERROR_INJECTION(__add_to_page_cache_locked, ERRNO);
-- 
2.26.2