Squashfs: implement .readpages()

Squashfs does not implement .readpages(), so the kernel just repeatedly
calls .readpage().

The readpages function tries to pack as much pages as possible in the
same page actor so that only 1 read request is issued.

Now that the read requests are asynchronous, the kernel can truly
prefetch pages using its readahead algorithm.

Signed-off-by: Adrien Schildknecht <adriens@google.com>
Change-Id: Ice70e029dc24526f61e4e5a1a902588be2212498

fs/squashfs/file.c

@@ -47,12 +47,16 @@
 #include <linux/string.h>
 #include <linux/pagemap.h>
 #include <linux/mutex.h>
+#include <linux/mm_inline.h>
 
 #include "squashfs_fs.h"
 #include "squashfs_fs_sb.h"
 #include "squashfs_fs_i.h"
 #include "squashfs.h"
 
+// Backported from 4.5
+#define lru_to_page(head) (list_entry((head)->prev, struct page, lru))
+
 /*
  * Locate cache slot in range [offset, index] for specified inode.  If
  * there's more than one return the slot closest to index.
@@ -438,6 +442,21 @@ static int squashfs_readpage_fragment(struct page *page)
 	return res;
 }
 
+static int squashfs_readpages_fragment(struct page *page,
+	struct list_head *readahead_pages, struct address_space *mapping)
+{
+	if (!page) {
+		page = lru_to_page(readahead_pages);
+		list_del(&page->lru);
+		if (add_to_page_cache_lru(page, mapping, page->index,
+			mapping_gfp_constraint(mapping, GFP_KERNEL))) {
+			put_page(page);
+			return 0;
+		}
+	}
+	return squashfs_readpage_fragment(page);
+}
+
 static int squashfs_readpage_sparse(struct page *page, int index, int file_end)
 {
 	struct inode *inode = page->mapping->host;
@@ -450,54 +469,105 @@ static int squashfs_readpage_sparse(struct page *page, int index, int file_end)
 	return 0;
 }
 
-static int squashfs_readpage(struct file *file, struct page *page)
+static int squashfs_readpages_sparse(struct page *page,
+	struct list_head *readahead_pages, int index, int file_end,
+	struct address_space *mapping)
 {
-	struct inode *inode = page->mapping->host;
+	if (!page) {
+		page = lru_to_page(readahead_pages);
+		list_del(&page->lru);
+		if (add_to_page_cache_lru(page, mapping, page->index,
+			mapping_gfp_constraint(mapping, GFP_KERNEL))) {
+			put_page(page);
+			return 0;
+		}
+	}
+	return squashfs_readpage_sparse(page, index, file_end);
+}
+
+static int __squashfs_readpages(struct file *file, struct page *page,
+	struct list_head *readahead_pages, unsigned int nr_pages,
+	struct address_space *mapping)
+{
+	struct inode *inode = mapping->host;
 	struct squashfs_sb_info *msblk = inode->i_sb->s_fs_info;
-	int index = page->index >> (msblk->block_log - PAGE_CACHE_SHIFT);
 	int file_end = i_size_read(inode) >> msblk->block_log;
 	int res;
-	void *pageaddr;
+
+	do {
+		struct page *cur_page = page ? page
+					     : lru_to_page(readahead_pages);
+		int page_index = cur_page->index;
+		int index = page_index >> (msblk->block_log - PAGE_CACHE_SHIFT);
+
+		if (page_index >= ((i_size_read(inode) + PAGE_CACHE_SIZE - 1) >>
+						PAGE_CACHE_SHIFT))
+			return 1;
+
+		if (index < file_end || squashfs_i(inode)->fragment_block ==
+						SQUASHFS_INVALID_BLK) {
+			u64 block = 0;
+			int bsize = read_blocklist(inode, index, &block);
+
+			if (bsize < 0)
+				return -1;
+
+			if (bsize == 0) {
+				res = squashfs_readpages_sparse(page,
+					readahead_pages, index, file_end,
+					mapping);
+			} else {
+				res = squashfs_readpages_block(page,
+					readahead_pages, &nr_pages, mapping,
+					page_index, block, bsize);
+			}
+		} else {
+			res = squashfs_readpages_fragment(page,
+				readahead_pages, mapping);
+		}
+		if (res)
+			return 0;
+		page = NULL;
+	} while (readahead_pages && !list_empty(readahead_pages));
+
+	return 0;
+}
+
+static int squashfs_readpage(struct file *file, struct page *page)
+{
+	int ret;
 
 	TRACE("Entered squashfs_readpage, page index %lx, start block %llx\n",
-				page->index, squashfs_i(inode)->start);
+	      page->index, squashfs_i(page->mapping->host)->start);
 
-	if (page->index >= ((i_size_read(inode) + PAGE_CACHE_SIZE - 1) >>
-					PAGE_CACHE_SHIFT))
-		goto out;
+	get_page(page);
 
-	if (index < file_end || squashfs_i(inode)->fragment_block ==
-					SQUASHFS_INVALID_BLK) {
-		u64 block = 0;
-		int bsize = read_blocklist(inode, index, &block);
-		if (bsize < 0)
-			goto error_out;
-
-		if (bsize == 0)
-			res = squashfs_readpage_sparse(page, index, file_end);
+	ret = __squashfs_readpages(file, page, NULL, 1, page->mapping);
+	if (ret) {
+		flush_dcache_page(page);
+		if (ret < 0)
+			SetPageError(page);
 		else
-			res = squashfs_readpage_block(page, block, bsize);
-	} else
-		res = squashfs_readpage_fragment(page);
+			SetPageUptodate(page);
+		zero_user_segment(page, 0, PAGE_CACHE_SIZE);
+		unlock_page(page);
+		put_page(page);
+	}
 
-	if (!res)
-		return 0;
-
-error_out:
-	SetPageError(page);
-out:
-	pageaddr = kmap_atomic(page);
-	memset(pageaddr, 0, PAGE_CACHE_SIZE);
-	kunmap_atomic(pageaddr);
-	flush_dcache_page(page);
-	if (!PageError(page))
-		SetPageUptodate(page);
-	unlock_page(page);
+	return 0;
+}
 
+static int squashfs_readpages(struct file *file, struct address_space *mapping,
+			      struct list_head *pages, unsigned int nr_pages)
+{
+	TRACE("Entered squashfs_readpages, %u pages, first page index %lx\n",
+		nr_pages, lru_to_page(pages)->index);
+	__squashfs_readpages(file, NULL, pages, nr_pages, mapping);
 	return 0;
 }
 
 
 const struct address_space_operations squashfs_aops = {
-	.readpage = squashfs_readpage
+	.readpage = squashfs_readpage,
+	.readpages = squashfs_readpages,
 };

fs/squashfs/file_direct.c

@@ -13,6 +13,7 @@
 #include <linux/string.h>
 #include <linux/pagemap.h>
 #include <linux/mutex.h>
+#include <linux/mm_inline.h>
 
 #include "squashfs_fs.h"
 #include "squashfs_fs_sb.h"
@@ -20,6 +21,9 @@
 #include "squashfs.h"
 #include "page_actor.h"
 
+// Backported from 4.5
+#define lru_to_page(head) (list_entry((head)->prev, struct page, lru))
+
 static void release_actor_pages(struct page **page, int pages, int error)
 {
 	int i;
@@ -45,23 +49,40 @@ static void release_actor_pages(struct page **page, int pages, int error)
  * page cache pages appropriately within the decompressor
  */
 static struct squashfs_page_actor *actor_from_page_cache(
-	struct page *target_page, int start_index, int nr_pages)
+	unsigned int actor_pages, struct page *target_page,
+	struct list_head *rpages, unsigned int *nr_pages, int start_index,
+	struct address_space *mapping)
 {
-	int i, n;
 	struct page **page;
 	struct squashfs_page_actor *actor;
+	int i, n;
+	gfp_t gfp = mapping_gfp_constraint(mapping, GFP_KERNEL);
 
-	page = kmalloc_array(nr_pages, sizeof(void *), GFP_KERNEL);
+	page = kmalloc_array(actor_pages, sizeof(void *), GFP_KERNEL);
 	if (!page)
 		return NULL;
 
-	/* Try to grab all the pages covered by the SquashFS block */
-	for (i = 0, n = start_index; i < nr_pages; i++, n++) {
-		if (target_page->index == n) {
+	for (i = 0, n = start_index; i < actor_pages; i++, n++) {
+		if (target_page == NULL && rpages && !list_empty(rpages)) {
+			struct page *cur_page = lru_to_page(rpages);
+
+			if (cur_page->index < start_index + actor_pages) {
+				list_del(&cur_page->lru);
+				--(*nr_pages);
+				if (add_to_page_cache_lru(cur_page, mapping,
+							  cur_page->index, gfp))
+					put_page(cur_page);
+				else
+					target_page = cur_page;
+			} else
+				rpages = NULL;
+		}
+
+		if (target_page && target_page->index == n) {
 			page[i] = target_page;
+			target_page = NULL;
 		} else {
-			page[i] = grab_cache_page_nowait(target_page->mapping,
-							 n);
+			page[i] = grab_cache_page_nowait(mapping, n);
 			if (page[i] == NULL)
 				continue;
 		}
@@ -73,39 +94,42 @@ static struct squashfs_page_actor *actor_from_page_cache(
 		}
 	}
 
-	actor = squashfs_page_actor_init(page, nr_pages, 0,
+	actor = squashfs_page_actor_init(page, actor_pages, 0,
 			release_actor_pages);
 	if (!actor) {
-		release_actor_pages(page, nr_pages, -ENOMEM);
+		release_actor_pages(page, actor_pages, -ENOMEM);
 		kfree(page);
 		return NULL;
 	}
 	return actor;
 }
 
-/* Read separately compressed datablock directly into page cache */
-int squashfs_readpage_block(struct page *target_page, u64 block, int bsize)
+int squashfs_readpages_block(struct page *target_page,
+			     struct list_head *readahead_pages,
+			     unsigned int *nr_pages,
+			     struct address_space *mapping,
+			     int page_index, u64 block, int bsize)
 
 {
-	struct inode *inode = target_page->mapping->host;
+	struct squashfs_page_actor *actor;
+	struct inode *inode = mapping->host;
 	struct squashfs_sb_info *msblk = inode->i_sb->s_fs_info;
-
 	int file_end = (i_size_read(inode) - 1) >> PAGE_CACHE_SHIFT;
 	int mask = (1 << (msblk->block_log - PAGE_CACHE_SHIFT)) - 1;
-	int start_index = target_page->index & ~mask;
+	int start_index = page_index & ~mask;
 	int end_index = start_index | mask;
-	int pages, res = -ENOMEM;
-	struct squashfs_page_actor *actor;
+	int actor_pages, res;
 
 	if (end_index > file_end)
 		end_index = file_end;
-	pages = end_index - start_index + 1;
+	actor_pages = end_index - start_index + 1;
 
-	actor = actor_from_page_cache(target_page, start_index, pages);
+	actor = actor_from_page_cache(actor_pages, target_page,
+				      readahead_pages, nr_pages, start_index,
+				      mapping);
 	if (!actor)
 		return -ENOMEM;
 
-	get_page(target_page);
 	res = squashfs_read_data_async(inode->i_sb, block, bsize, NULL,
 				       actor);
 	return res < 0 ? res : 0;

fs/squashfs/squashfs.h

@@ -76,8 +76,9 @@ extern __le64 *squashfs_read_fragment_index_table(struct super_block *,
 void squashfs_copy_cache(struct page *, struct squashfs_cache_entry *, int,
 				int);
 
-/* file_xxx.c */
-extern int squashfs_readpage_block(struct page *, u64, int);
+/* file_direct.c */
+extern int squashfs_readpages_block(struct page *, struct list_head *,
+	unsigned int *, struct address_space *, int, u64, int);
 
 /* id.c */
 extern int squashfs_get_id(struct super_block *, unsigned int, unsigned int *);