Date:	Mon, 11 Aug 2008 15:53:07 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	cmm@...ibm.com, tytso@....edu, sandeen@...hat.com
Cc:	linux-ext4@...r.kernel.org,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: [PATCH -v2] ext4: Rework the ext4_da_writepages

With the below changes we reserve credit needed to insert only one extent
resulting from a call to single get_block. That make 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 <aneesh.kumar@...ux.vnet.ibm.com>
---
 fs/ext4/inode.c |  200 ++++++++++++++++++++++++++++--------------------------
 1 files changed, 104 insertions(+), 96 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 7a66bba..26e30ed 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -41,6 +41,8 @@
 #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)
 {
@@ -1604,11 +1606,13 @@ static void ext4_da_page_release_reservation(struct page *page,
 	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
@@ -1623,18 +1627,11 @@ static void ext4_da_page_release_reservation(struct page *page,
 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;
@@ -1652,8 +1649,9 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd)
 				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
@@ -1664,9 +1662,6 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd)
 		}
 		pagevec_release(&pvec);
 	}
-	if (mpd_pp.bio)
-		mpage_bio_submit(WRITE, mpd_pp.bio);
-
 	return ret;
 }
 
@@ -1689,7 +1684,7 @@ static void mpage_put_bnr_to_bhs(struct mpage_da_data *mpd, sector_t logical,
 	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;
 
@@ -1774,13 +1769,11 @@ static inline void __unmap_underlying_blocks(struct inode *inode,
  *
  * 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;
 
@@ -1790,35 +1783,32 @@ static void mpage_da_map_blocks(struct mpage_da_data *mpd)
 	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 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 (buffer_new(&new))
-			__unmap_underlying_blocks(mpd->inode, &new);
+	if (buffer_new(&new))
+		__unmap_underlying_blocks(mpd->inode, &new);
 
-		/*
-		 * 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 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);
 
-		/* go for the remaining blocks */
-		next += new.b_size >> mpd->inode->i_blkbits;
-		remain -= new.b_size;
-	}
+	return;
 }
 
 #define BH_FLAGS ((1 << BH_Uptodate) | (1 << BH_Mapped) | \
@@ -1864,13 +1854,9 @@ static void mpage_add_bh_to_extent(struct mpage_da_data *mpd,
 	 * 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;
 }
 
 /*
@@ -1890,17 +1876,35 @@ static int __mpage_da_writepage(struct page *page,
 	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;
 		}
 
 		/*
@@ -1931,6 +1935,8 @@ static int __mpage_da_writepage(struct page *page,
 		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
@@ -1939,8 +1945,12 @@ static int __mpage_da_writepage(struct page *page,
 		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);
 	}
@@ -1959,22 +1969,13 @@ static int __mpage_da_writepage(struct page *page,
  *
  * 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)
@@ -1988,17 +1989,22 @@ static int mpage_da_writepages(struct address_space *mapping,
 	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;
 }
 
@@ -2210,10 +2216,7 @@ static int ext4_da_writepages(struct address_space *mapping,
 	int ret = 0;
 	long to_write;
 	loff_t range_start = 0;
-	int blocks_per_page = PAGE_CACHE_SIZE >> inode->i_blkbits;
-	int max_credit_blocks = ext4_journal_max_transaction_buffers(inode);
-	int need_credits_per_page =  ext4_writepages_trans_blocks(inode, 1);
-	int max_writeback_pages = (max_credit_blocks / blocks_per_page) / need_credits_per_page;
+	long pages_skipped = 0;
 
 	/*
 	 * No pages to write? This is mainly a kludge to avoid starting
@@ -2223,45 +2226,35 @@ static int ext4_da_writepages(struct address_space *mapping,
 	if (!mapping->nrpages || !mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
 		return 0;
 
-	if (wbc->nr_to_write > mapping->nrpages)
-		wbc->nr_to_write = mapping->nrpages;
-
-	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) {
-		/*
-		 * set the max dirty pages could be write at a time
-		 * to fit into the reserved transaction credits
-		 */
-		if (wbc->nr_to_write > max_writeback_pages)
-			wbc->nr_to_write = max_writeback_pages;
+	range_start =  wbc->range_start;
+	pages_skipped = wbc->pages_skipped;
+
+restart_loop:
+	to_write = wbc->nr_to_write;
+	while (!ret && to_write > 0) {
 
 		/*
-		 * Estimate the worse case needed credits to write out
-		 * to_write pages
+		 * we  insert one extent at a time. So we need
+		 * credit needed for single extent allocation.
+		 * journalled mode is currently not supported
+		 * by delalloc
 		 */
-		needed_blocks = ext4_writepages_trans_blocks(inode,
-							     wbc->nr_to_write);
-		while (needed_blocks > max_credit_blocks) {
-			wbc->nr_to_write--;
-			needed_blocks = ext4_writepages_trans_blocks(inode,
-							     wbc->nr_to_write);
-		}
+		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 "ext4_da_writepages: jbd2_start: "
-			       "%ld pages, ino %lu; err %d\n",
+			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;
@@ -2284,7 +2277,14 @@ static int ext4_da_writepages(struct address_space *mapping,
 		ret = mpage_da_writepages(mapping, wbc,
 						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
@@ -2296,10 +2296,18 @@ static int ext4_da_writepages(struct address_space *mapping,
 		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;
 }
 
-- 
1.6.0.rc0.42.g186458.dirty

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