squashfs: implement readahead

Implement readahead callback for squashfs.  It will read datablocks which
cover pages in readahead request.  For a few cases it will not mark page
as uptodate, including:

- file end is 0.
- zero filled blocks.
- current batch of pages isn't in the same datablock.
- decompressor error.

Otherwise pages will be marked as uptodate.  The unhandled pages will be
updated by readpage later.

Link: https://lkml.kernel.org/r/20220617083810.337573-4-hsinyi@chromium.org
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Reported-by: Matthew Wilcox <willy@infradead.org>
Reported-by: Phillip Lougher <phillip@squashfs.org.uk>
Reported-by: Xiongwei Song <Xiongwei.Song@windriver.com>
Reported-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Hou Tao <houtao1@huawei.com>
Cc: kernel test robot <lkp@intel.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Miao Xie <miaoxie@huawei.com>
Cc: Zhang Yi <yi.zhang@huawei.com>
Cc: Zheng Liang <zhengliang6@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/squashfs/file.c

@@ -39,6 +39,7 @@
 #include "squashfs_fs_sb.h"
 #include "squashfs_fs_i.h"
 #include "squashfs.h"
+#include "page_actor.h"
 
 /*
  * Locate cache slot in range [offset, index] for specified inode.  If
@@ -495,7 +496,96 @@ out:
 	return 0;
 }
 
+static void squashfs_readahead(struct readahead_control *ractl)
+{
+	struct inode *inode = ractl->mapping->host;
+	struct squashfs_sb_info *msblk = inode->i_sb->s_fs_info;
+	size_t mask = (1UL << msblk->block_log) - 1;
+	unsigned short shift = msblk->block_log - PAGE_SHIFT;
+	loff_t start = readahead_pos(ractl) & ~mask;
+	size_t len = readahead_length(ractl) + readahead_pos(ractl) - start;
+	struct squashfs_page_actor *actor;
+	unsigned int nr_pages = 0;
+	struct page **pages;
+	int i, file_end = i_size_read(inode) >> msblk->block_log;
+	unsigned int max_pages = 1UL << shift;
+
+	readahead_expand(ractl, start, (len | mask) + 1);
+
+	if (file_end == 0)
+		return;
+
+	pages = kmalloc_array(max_pages, sizeof(void *), GFP_KERNEL);
+	if (!pages)
+		return;
+
+	for (;;) {
+		pgoff_t index;
+		int res, bsize;
+		u64 block = 0;
+		unsigned int expected;
+
+		nr_pages = __readahead_batch(ractl, pages, max_pages);
+		if (!nr_pages)
+			break;
+
+		if (readahead_pos(ractl) >= i_size_read(inode))
+			goto skip_pages;
+
+		index = pages[0]->index >> shift;
+		if ((pages[nr_pages - 1]->index >> shift) != index)
+			goto skip_pages;
+
+		expected = index == file_end ?
+			   (i_size_read(inode) & (msblk->block_size - 1)) :
+			    msblk->block_size;
+
+		bsize = read_blocklist(inode, index, &block);
+		if (bsize == 0)
+			goto skip_pages;
+
+		actor = squashfs_page_actor_init_special(msblk, pages, nr_pages,
+							 expected);
+		if (!actor)
+			goto skip_pages;
+
+		res = squashfs_read_data(inode->i_sb, block, bsize, NULL, actor);
+
+		kfree(actor);
+
+		if (res == expected) {
+			int bytes;
+
+			/* Last page (if present) may have trailing bytes not filled */
+			bytes = res % PAGE_SIZE;
+			if (pages[nr_pages - 1]->index == file_end && bytes)
+				memzero_page(pages[nr_pages - 1], bytes,
+					     PAGE_SIZE - bytes);
+
+			for (i = 0; i < nr_pages; i++) {
+				flush_dcache_page(pages[i]);
+				SetPageUptodate(pages[i]);
+			}
+		}
+
+		for (i = 0; i < nr_pages; i++) {
+			unlock_page(pages[i]);
+			put_page(pages[i]);
+		}
+	}
+
+	kfree(pages);
+	return;
+
+skip_pages:
+	for (i = 0; i < nr_pages; i++) {
+		unlock_page(pages[i]);
+		put_page(pages[i]);
+	}
+	kfree(pages);
+}
 
 const struct address_space_operations squashfs_aops = {
-	.read_folio = squashfs_read_folio
+	.read_folio = squashfs_read_folio,
+	.readahead = squashfs_readahead
 };