mm/shmem: return head page from find_lock_entry

Convert shmem_getpage_gfp() (the only remaining caller of
find_lock_entry()) to cope with a head page being returned instead of
the subpage for the index.

[willy@infradead.org: fix BUG()s]
  Link https://lore.kernel.org/linux-mm/20200912032042.GA6583@casper.infradead.org/

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: William Kucharski <william.kucharski@oracle.com>
Link: https://lkml.kernel.org/r/20200910183318.20139-8-willy@infradead.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/pagemap.h

@@ -372,6 +372,15 @@ static inline struct page *grab_cache_page_nowait(struct address_space *mapping,
 			mapping_gfp_mask(mapping));
 }
 
+/* Does this page contain this index? */
+static inline bool thp_contains(struct page *head, pgoff_t index)
+{
+	/* HugeTLBfs indexes the page cache in units of hpage_size */
+	if (PageHuge(head))
+		return head->index == index;
+	return page_index(head) == (index & ~(thp_nr_pages(head) - 1UL));
+}
+
 /*
  * Given the page we found in the page cache, return the page corresponding
  * to this index in the file

mm/filemap.c

@@ -1692,37 +1692,34 @@ out:
 }
 
 /**
- * find_lock_entry - locate, pin and lock a page cache entry
- * @mapping: the address_space to search
- * @offset: the page cache index
+ * find_lock_entry - Locate and lock a page cache entry.
+ * @mapping: The address_space to search.
+ * @index: The page cache index.
  *
- * Looks up the page cache slot at @mapping & @offset.  If there is a
- * page cache page, it is returned locked and with an increased
- * refcount.
+ * Looks up the page at @mapping & @index.  If there is a page in the
+ * cache, the head page is returned locked and with an increased refcount.
  *
  * If the slot holds a shadow entry of a previously evicted page, or a
  * swap entry from shmem/tmpfs, it is returned.
  *
- * find_lock_entry() may sleep.
- *
- * Return: the found page or shadow entry, %NULL if nothing is found.
+ * Context: May sleep.
+ * Return: The head page or shadow entry, %NULL if nothing is found.
  */
-struct page *find_lock_entry(struct address_space *mapping, pgoff_t offset)
+struct page *find_lock_entry(struct address_space *mapping, pgoff_t index)
 {
 	struct page *page;
 
 repeat:
-	page = find_get_entry(mapping, offset);
+	page = find_get_entry(mapping, index);
 	if (page && !xa_is_value(page)) {
 		lock_page(page);
 		/* Has the page been truncated? */
-		if (unlikely(page_mapping(page) != mapping)) {
+		if (unlikely(page->mapping != mapping)) {
 			unlock_page(page);
 			put_page(page);
 			goto repeat;
 		}
-		page = find_subpage(page, offset);
-		VM_BUG_ON_PAGE(page_to_pgoff(page) != offset, page);
+		VM_BUG_ON_PAGE(!thp_contains(page, index), page);
 	}
 	return page;
 }

mm/shmem.c

@@ -1830,6 +1830,8 @@ repeat:
 		return error;
 	}
 
+	if (page)
+		hindex = page->index;
 	if (page && sgp == SGP_WRITE)
 		mark_page_accessed(page);
 
@@ -1840,11 +1842,10 @@ repeat:
 		unlock_page(page);
 		put_page(page);
 		page = NULL;
+		hindex = index;
 	}
-	if (page || sgp == SGP_READ) {
-		*pagep = page;
-		return 0;
-	}
+	if (page || sgp == SGP_READ)
+		goto out;
 
 	/*
 	 * Fast cache lookup did not find it:
@@ -1969,14 +1970,13 @@ clear:
 	 * it now, lest undo on failure cancel our earlier guarantee.
 	 */
 	if (sgp != SGP_WRITE && !PageUptodate(page)) {
-		struct page *head = compound_head(page);
 		int i;
 
-		for (i = 0; i < compound_nr(head); i++) {
-			clear_highpage(head + i);
-			flush_dcache_page(head + i);
+		for (i = 0; i < compound_nr(page); i++) {
+			clear_highpage(page + i);
+			flush_dcache_page(page + i);
 		}
-		SetPageUptodate(head);
+		SetPageUptodate(page);
 	}
 
 	/* Perhaps the file has been truncated since we checked */
@@ -1992,6 +1992,7 @@ clear:
 		error = -EINVAL;
 		goto unlock;
 	}
+out:
 	*pagep = page + index - hindex;
 	return 0;