[PATCH v5 3/3] squashfs: implement readahead

[Hsin-Yi Wang <hsinyi@chromium.org>]

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.

Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.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: Marek Szyprowski <m.szyprowski@samsung.com>
Reported-by: Andrew Morton <akpm@linux-foundation.org>
---
v4->v5:
- Handle short file cases reported by Marek and Matthew.
- Fix checkpatch error reported by Andrew.

v4: https://lore.kernel.org/lkml/20220601103922.1338320-4-hsinyi@chromium.org/
v3: https://lore.kernel.org/lkml/20220523065909.883444-4-hsinyi@chromium.org/
v2: https://lore.kernel.org/lkml/20220517082650.2005840-4-hsinyi@chromium.org/
v1: https://lore.kernel.org/lkml/20220516105100.1412740-3-hsinyi@chromium.org/
---
 fs/squashfs/file.c | 124 ++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 123 insertions(+), 1 deletion(-)

diff --git a/fs/squashfs/file.c b/fs/squashfs/file.c
index a8e495d8eb86..fbd096cd15f4 100644
--- a/fs/squashfs/file.c
+++ b/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 +496,128 @@ static int squashfs_read_folio(struct file *file, struct folio *folio)
 	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;
+
+	actor = squashfs_page_actor_init_special(pages, max_pages, 0);
+	if (!actor)
+		goto out;
+
+	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;
+
+		if (nr_pages < max_pages) {
+			struct squashfs_cache_entry *buffer;
+			unsigned int block_mask = max_pages - 1;
+			int offset = pages[0]->index - (pages[0]->index & ~block_mask);
+
+			buffer = squashfs_get_datablock(inode->i_sb, block,
+							bsize);
+			if (buffer->error) {
+				squashfs_cache_put(buffer);
+				goto skip_pages;
+			}
+
+			expected -= offset * PAGE_SIZE;
+			for (i = 0; i < nr_pages && expected > 0; i++,
+						expected -= PAGE_SIZE, offset++) {
+				int avail = min_t(int, expected, PAGE_SIZE);
+
+				squashfs_fill_page(pages[i], buffer,
+						offset * PAGE_SIZE, avail);
+				unlock_page(pages[i]);
+			}
+
+			squashfs_cache_put(buffer);
+			continue;
+		}
+
+		res = squashfs_read_data(inode->i_sb, block, bsize, NULL,
+					 actor);
+
+		if (res == expected) {
+			int bytes;
+
+			/* Last page may have trailing bytes not filled */
+			bytes = res % PAGE_SIZE;
+			if (bytes) {
+				void *pageaddr;
+
+				pageaddr = kmap_atomic(pages[nr_pages - 1]);
+				memset(pageaddr + bytes, 0, PAGE_SIZE - bytes);
+				kunmap_atomic(pageaddr);
+			}
+
+			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(actor);
+	kfree(pages);
+	return;
+
+skip_pages:
+	for (i = 0; i < nr_pages; i++) {
+		unlock_page(pages[i]);
+		put_page(pages[i]);
+	}
+
+	kfree(actor);
+out:
+	kfree(pages);
+}
 
 const struct address_space_operations squashfs_aops = {
-	.read_folio = squashfs_read_folio
+	.read_folio = squashfs_read_folio,
+	.readahead = squashfs_readahead
 };
-- 
2.36.1.255.ge46751e96f-goog