squashfs: don't use intermediate buffer if pages missing

Now that the "page actor" can handle missing pages, we don't have to fall
back to using an intermediate buffer in Squashfs_readpage_block() if all
the pages necessary can't be obtained.

Link: https://lkml.kernel.org/r/20220611032133.5743-3-phillip@squashfs.org.uk
Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
Cc: Hsin-Yi Wang <hsinyi@chromium.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Xiongwei Song <Xiongwei.Song@windriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/squashfs/file_direct.c

@@ -18,9 +18,6 @@
 #include "squashfs.h"
 #include "page_actor.h"
 
-static int squashfs_read_cache(struct page *target_page, u64 block, int bsize,
-	int pages, struct page **page, int bytes);
-
 /* Read separately compressed datablock directly into page cache */
 int squashfs_readpage_block(struct page *target_page, u64 block, int bsize,
 	int expected)
@@ -33,7 +30,7 @@ int squashfs_readpage_block(struct page *target_page, u64 block, int bsize,
 	int mask = (1 << (msblk->block_log - PAGE_SHIFT)) - 1;
 	int start_index = target_page->index & ~mask;
 	int end_index = start_index | mask;
-	int i, n, pages, missing_pages, bytes, res = -ENOMEM;
+	int i, n, pages, bytes, res = -ENOMEM;
 	struct page **page;
 	struct squashfs_page_actor *actor;
 	void *pageaddr;
@@ -48,44 +45,29 @@ int squashfs_readpage_block(struct page *target_page, u64 block, int bsize,
 		return res;
 
 	/* Try to grab all the pages covered by the Squashfs block */
-	for (missing_pages = 0, i = 0, n = start_index; i < pages; i++, n++) {
+	for (i = 0, n = start_index; n <= end_index; n++) {
 		page[i] = (n == target_page->index) ? target_page :
 			grab_cache_page_nowait(target_page->mapping, n);
 
-		if (page[i] == NULL) {
-			missing_pages++;
+		if (page[i] == NULL)
 			continue;
-		}
 
 		if (PageUptodate(page[i])) {
 			unlock_page(page[i]);
 			put_page(page[i]);
-			page[i] = NULL;
-			missing_pages++;
+			continue;
 		}
-	}
-
-	if (missing_pages) {
-		/*
-		 * Couldn't get one or more pages, this page has either
-		 * been VM reclaimed, but others are still in the page cache
-		 * and uptodate, or we're racing with another thread in
-		 * squashfs_readpage also trying to grab them.  Fall back to
-		 * using an intermediate buffer.
-		 */
-		res = squashfs_read_cache(target_page, block, bsize, pages,
-							page, expected);
-		if (res < 0)
-			goto mark_errored;
 
-		goto out;
+		i++;
 	}
 
+	pages = i;
+
 	/*
 	 * Create a "page actor" which will kmap and kunmap the
 	 * page cache pages appropriately within the decompressor
 	 */
-	actor = squashfs_page_actor_init_special(msblk, page, pages, 0);
+	actor = squashfs_page_actor_init_special(msblk, page, pages, expected);
 	if (actor == NULL)
 		goto out;
 
@@ -102,12 +84,12 @@ int squashfs_readpage_block(struct page *target_page, u64 block, int bsize,
 		goto mark_errored;
 	}
 
-	/* Last page may have trailing bytes not filled */
+	/* Last page (if present) may have trailing bytes not filled */
 	bytes = res % PAGE_SIZE;
-	if (bytes) {
-		pageaddr = kmap_atomic(page[pages - 1]);
+	if (page[pages - 1]->index == end_index && bytes) {
+		pageaddr = kmap_local_page(page[pages - 1]);
 		memset(pageaddr + bytes, 0, PAGE_SIZE - bytes);
-		kunmap_atomic(pageaddr);
+		kunmap_local(pageaddr);
 	}
 
 	/* Mark pages as uptodate, unlock and release */
@@ -140,36 +122,3 @@ int squashfs_readpage_block(struct page *target_page, u64 block, int bsize,
 	kfree(page);
 	return res;
 }
-
-
-static int squashfs_read_cache(struct page *target_page, u64 block, int bsize,
-	int pages, struct page **page, int bytes)
-{
-	struct inode *i = target_page->mapping->host;
-	struct squashfs_cache_entry *buffer = squashfs_get_datablock(i->i_sb,
-						 block, bsize);
-	int res = buffer->error, n, offset = 0;
-
-	if (res) {
-		ERROR("Unable to read page, block %llx, size %x\n", block,
-			bsize);
-		goto out;
-	}
-
-	for (n = 0; n < pages && bytes > 0; n++,
-			bytes -= PAGE_SIZE, offset += PAGE_SIZE) {
-		int avail = min_t(int, bytes, PAGE_SIZE);
-
-		if (page[n] == NULL)
-			continue;
-
-		squashfs_fill_page(page[n], buffer, offset, avail);
-		unlock_page(page[n]);
-		if (page[n] != target_page)
-			put_page(page[n]);
-	}
-
-out:
-	squashfs_cache_put(buffer);
-	return res;
-}