ext4: Rework the ext4_da_writepages() function

With the below changes we reserve credit needed to insert only one
extent resulting from a call to single get_block. This makes sure we
don't take too much journal credits during writeout. We also don't
limit the pages to write. That means we loop through the dirty pages
building largest possible contiguous block request. Then we issue a
single get_block request. We may get less block that we requested. If
so we would end up not mapping some of the buffer_heads. That means
those buffer_heads are still marked delay. Later in the writepage
callback via __mpage_writepage we redirty those pages.

We should also not limit/throttle wbc->nr_to_write in the filesystem
writepages callback. That cause wrong behaviour in
generic_sync_sb_inodes caused by wbc->nr_to_write being <= 0

Signed-off-by: Aneesh Kumar K.V <[e-mail]>
Reviewed-by: Mingming Cao <[e-mail]>
Signed-off-by: "Theodore Ts'o" <[e-mail]>

fs/ext4/inode.c

···
 #include "acl.h"
 #include "ext4_extents.h"
 
+#define MPAGE_DA_EXTENT_TAIL 0x01
+
 static inline int ext4_begin_ordered_truncate(struct inode *inode,
 					      loff_t new_size)
 {
···
 	unsigned long first_page, next_page;	/* extent of pages */
 	get_block_t *get_block;
 	struct writeback_control *wbc;
+	int io_done;
+	long pages_written;
 };
 
 /*
  * mpage_da_submit_io - walks through extent of pages and try to write
- * them with __mpage_writepage()
+ * them with writepage() call back
  *
  * @mpd->inode: inode
  * @mpd->first_page: first page of the extent
···
 static int mpage_da_submit_io(struct mpage_da_data *mpd)
 {
 	struct address_space *mapping = mpd->inode->i_mapping;
-	struct mpage_data mpd_pp = {
-		.bio = NULL,
-		.last_block_in_bio = 0,
-		.get_block = mpd->get_block,
-		.use_writepage = 1,
-	};
 	int ret = 0, err, nr_pages, i;
 	unsigned long index, end;
 	struct pagevec pvec;
 
 	BUG_ON(mpd->next_page <= mpd->first_page);
-
 	pagevec_init(&pvec, 0);
 	index = mpd->first_page;
 	end = mpd->next_page - 1;
···
 				break;
 			index++;
 
-			err = __mpage_writepage(page, mpd->wbc, &mpd_pp);
-
+			err = mapping->a_ops->writepage(page, mpd->wbc);
+			if (!err)
+				mpd->pages_written++;
 			/*
 			 * In error case, we have to continue because
 			 * remaining pages are still locked
···
 		}
 		pagevec_release(&pvec);
 	}
-	if (mpd_pp.bio)
-		mpage_bio_submit(WRITE, mpd_pp.bio);
-
 	return ret;
 }
 
···
 	int blocks = exbh->b_size >> inode->i_blkbits;
 	sector_t pblock = exbh->b_blocknr, cur_logical;
 	struct buffer_head *head, *bh;
-	unsigned long index, end;
+	pgoff_t index, end;
 	struct pagevec pvec;
 	int nr_pages, i;
 
···
  *
  * The function skips space we know is already mapped to disk blocks.
  *
- * The function ignores errors ->get_block() returns, thus real
- * error handling is postponed to __mpage_writepage()
  */
 static void mpage_da_map_blocks(struct mpage_da_data *mpd)
 {
+	int err = 0;
 	struct buffer_head *lbh = &mpd->lbh;
-	int err = 0, remain = lbh->b_size;
 	sector_t next = lbh->b_blocknr;
 	struct buffer_head new;
 
···
 	if (buffer_mapped(lbh) && !buffer_delay(lbh))
 		return;
 
-	while (remain) {
-		new.b_state = lbh->b_state;
-		new.b_blocknr = 0;
-		new.b_size = remain;
-		err = mpd->get_block(mpd->inode, next, &new, 1);
-		if (err) {
-			/*
-			 * Rather than implement own error handling
-			 * here, we just leave remaining blocks
-			 * unallocated and try again with ->writepage()
-			 */
-			break;
-		}
-		BUG_ON(new.b_size == 0);
+	new.b_state = lbh->b_state;
+	new.b_blocknr = 0;
+	new.b_size = lbh->b_size;
 
-		if (buffer_new(&new))
-			__unmap_underlying_blocks(mpd->inode, &new);
+	/*
+	 * If we didn't accumulate anything
+	 * to write simply return
+	 */
+	if (!new.b_size)
+		return;
+	err = mpd->get_block(mpd->inode, next, &new, 1);
+	if (err)
+		return;
+	BUG_ON(new.b_size == 0);
 
-		/*
-		 * If blocks are delayed marked, we need to
-		 * put actual blocknr and drop delayed bit
-		 */
-		if (buffer_delay(lbh) || buffer_unwritten(lbh))
-			mpage_put_bnr_to_bhs(mpd, next, &new);
+	if (buffer_new(&new))
+		__unmap_underlying_blocks(mpd->inode, &new);
 
-		/* go for the remaining blocks */
-		next += new.b_size >> mpd->inode->i_blkbits;
-		remain -= new.b_size;
-	}
+	/*
+	 * If blocks are delayed marked, we need to
+	 * put actual blocknr and drop delayed bit
+	 */
+	if (buffer_delay(lbh) || buffer_unwritten(lbh))
+		mpage_put_bnr_to_bhs(mpd, next, &new);
+
+	return;
 }
 
 #define BH_FLAGS ((1 << BH_Uptodate) | (1 << BH_Mapped) | \
···
 	 * need to flush current  extent and start new one
 	 */
 	mpage_da_map_blocks(mpd);
-
-	/*
-	 * Now start a new extent
-	 */
-	lbh->b_size = bh->b_size;
-	lbh->b_state = bh->b_state & BH_FLAGS;
-	lbh->b_blocknr = logical;
+	mpage_da_submit_io(mpd);
+	mpd->io_done = 1;
+	return;
 }
 
 /*
···
 	struct buffer_head *bh, *head, fake;
 	sector_t logical;
 
+	if (mpd->io_done) {
+		/*
+		 * Rest of the page in the page_vec
+		 * redirty then and skip then. We will
+		 * try to to write them again after
+		 * starting a new transaction
+		 */
+		redirty_page_for_writepage(wbc, page);
+		unlock_page(page);
+		return MPAGE_DA_EXTENT_TAIL;
+	}
 	/*
 	 * Can we merge this page to current extent?
 	 */
 	if (mpd->next_page != page->index) {
 		/*
 		 * Nope, we can't. So, we map non-allocated blocks
-		 * and start IO on them using __mpage_writepage()
+		 * and start IO on them using writepage()
 		 */
 		if (mpd->next_page != mpd->first_page) {
 			mpage_da_map_blocks(mpd);
 			mpage_da_submit_io(mpd);
+			/*
+			 * skip rest of the page in the page_vec
+			 */
+			mpd->io_done = 1;
+			redirty_page_for_writepage(wbc, page);
+			unlock_page(page);
+			return MPAGE_DA_EXTENT_TAIL;
 		}
 
 		/*
···
 		set_buffer_dirty(bh);
 		set_buffer_uptodate(bh);
 		mpage_add_bh_to_extent(mpd, logical, bh);
+		if (mpd->io_done)
+			return MPAGE_DA_EXTENT_TAIL;
 	} else {
 		/*
 		 * Page with regular buffer heads, just add all dirty ones
···
 		bh = head;
 		do {
 			BUG_ON(buffer_locked(bh));
-			if (buffer_dirty(bh))
+			if (buffer_dirty(bh) &&
+				(!buffer_mapped(bh) || buffer_delay(bh))) {
 				mpage_add_bh_to_extent(mpd, logical, bh);
+				if (mpd->io_done)
+					return MPAGE_DA_EXTENT_TAIL;
+			}
 			logical++;
 		} while ((bh = bh->b_this_page) != head);
 	}
···
  *
  * This is a library function, which implements the writepages()
  * address_space_operation.
- *
- * In order to avoid duplication of logic that deals with partial pages,
- * multiple bio per page, etc, we find non-allocated blocks, allocate
- * them with minimal calls to ->get_block() and re-use __mpage_writepage()
- *
- * It's important that we call __mpage_writepage() only once for each
- * involved page, otherwise we'd have to implement more complicated logic
- * to deal with pages w/o PG_lock or w/ PG_writeback and so on.
- *
- * See comments to mpage_writepages()
  */
 static int mpage_da_writepages(struct address_space *mapping,
 			       struct writeback_control *wbc,
 			       get_block_t get_block)
 {
 	struct mpage_da_data mpd;
+	long to_write;
 	int ret;
 
 	if (!get_block)
···
 	mpd.first_page = 0;
 	mpd.next_page = 0;
 	mpd.get_block = get_block;
+	mpd.io_done = 0;
+	mpd.pages_written = 0;
+
+	to_write = wbc->nr_to_write;
 
 	ret = write_cache_pages(mapping, wbc, __mpage_da_writepage, &mpd);
 
 	/*
 	 * Handle last extent of pages
 	 */
-	if (mpd.next_page != mpd.first_page) {
+	if (!mpd.io_done && mpd.next_page != mpd.first_page) {
 		mpage_da_map_blocks(&mpd);
 		mpage_da_submit_io(&mpd);
 	}
 
+	wbc->nr_to_write = to_write - mpd.pages_written;
 	return ret;
 }
 
···
 #define EXT4_MAX_WRITEBACK_CREDITS    25
 
 static int ext4_da_writepages(struct address_space *mapping,
-				struct writeback_control *wbc)
+			      struct writeback_control *wbc)
 {
 	struct inode *inode = mapping->host;
 	handle_t *handle = NULL;
···
 	int ret = 0;
 	long to_write;
 	loff_t range_start = 0;
+	long pages_skipped = 0;
 
 	/*
 	 * No pages to write? This is mainly a kludge to avoid starting
 	 * a transaction for special inodes like journal inode on last iput()
 	 * because that could violate lock ordering on umount
 	 */
-	if (!mapping->nrpages)
+	if (!mapping->nrpages || !mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
 		return 0;
 
-	/*
-	 * Estimate the worse case needed credits to write out
-	 * EXT4_MAX_BUF_BLOCKS pages
-	 */
-	needed_blocks = EXT4_MAX_WRITEBACK_CREDITS;
-
-	to_write = wbc->nr_to_write;
-	if (!wbc->range_cyclic) {
+	if (!wbc->range_cyclic)
 		/*
 		 * If range_cyclic is not set force range_cont
 		 * and save the old writeback_index
 		 */
 		wbc->range_cont = 1;
-		range_start =  wbc->range_start;
-	}
 
-	while (!ret && to_write) {
+	range_start =  wbc->range_start;
+	pages_skipped = wbc->pages_skipped;
+
+restart_loop:
+	to_write = wbc->nr_to_write;
+	while (!ret && to_write > 0) {
+
+		/*
+		 * we  insert one extent at a time. So we need
+		 * credit needed for single extent allocation.
+		 * journalled mode is currently not supported
+		 * by delalloc
+		 */
+		BUG_ON(ext4_should_journal_data(inode));
+		needed_blocks = EXT4_DATA_TRANS_BLOCKS(inode->i_sb);
+
 		/* start a new transaction*/
 		handle = ext4_journal_start(inode, needed_blocks);
 		if (IS_ERR(handle)) {
 			ret = PTR_ERR(handle);
+			printk(KERN_EMERG "%s: jbd2_start: "
+			       "%ld pages, ino %lu; err %d\n", __func__,
+				wbc->nr_to_write, inode->i_ino, ret);
+			dump_stack();
 			goto out_writepages;
 		}
 		if (ext4_should_order_data(inode)) {
 			/*
 			 * With ordered mode we need to add
-			 * the inode to the journal handle
+			 * the inode to the journal handl
 			 * when we do block allocation.
 			 */
 			ret = ext4_jbd2_file_inode(handle, inode);
···
 				ext4_journal_stop(handle);
 				goto out_writepages;
 			}
-
 		}
-		/*
-		 * set the max dirty pages could be write at a time
-		 * to fit into the reserved transaction credits
-		 */
-		if (wbc->nr_to_write > EXT4_MAX_WRITEBACK_PAGES)
-			wbc->nr_to_write = EXT4_MAX_WRITEBACK_PAGES;
 
 		to_write -= wbc->nr_to_write;
 		ret = mpage_da_writepages(mapping, wbc,
-						ext4_da_get_block_write);
+					  ext4_da_get_block_write);
 		ext4_journal_stop(handle);
-		if (wbc->nr_to_write) {
+		if (ret == MPAGE_DA_EXTENT_TAIL) {
+			/*
+			 * got one extent now try with
+			 * rest of the pages
+			 */
+			to_write += wbc->nr_to_write;
+			ret = 0;
+		} else if (wbc->nr_to_write) {
 			/*
 			 * There is no more writeout needed
 			 * or we requested for a noblocking writeout
···
 		wbc->nr_to_write = to_write;
 	}
 
+	if (wbc->range_cont && (pages_skipped != wbc->pages_skipped)) {
+		/* We skipped pages in this loop */
+		wbc->range_start = range_start;
+		wbc->nr_to_write = to_write +
+				wbc->pages_skipped - pages_skipped;
+		wbc->pages_skipped = pages_skipped;
+		goto restart_loop;
+	}
+
 out_writepages:
 	wbc->nr_to_write = to_write;
-	if (range_start)
-		wbc->range_start = range_start;
+	wbc->range_start = range_start;
 	return ret;
 }