[PATCH v2 3/6] ext4: restart handle if credits are insufficient during allocating blocks

[Zhang Yi <yi.zhang@huaweicloud.com>]

From: Zhang Yi <yi.zhang@huawei.com>

After large folios are supported on ext4, writing back a sufficiently
large and discontinuous folio may consume a significant number of
journal credits, placing considerable strain on the journal. For
example, in a 20GB filesystem with 1K block size and 1MB journal size,
writing back a 2MB folio could require thousands of credits in the
worst-case scenario (when each block is discontinuous and distributed
across different block groups), potentially exceeding the journal size.
This issue can also occur in ext4_write_begin() and ext4_page_mkwrite()
when delalloc is not enabled.

Fix this by ensuring that there are sufficient journal credits before
allocating an extent in mpage_map_one_extent() and _ext4_get_block(). If
there are not enough credits, return -EAGAIN, exit the current mapping
loop, restart a new handle and a new transaction, and allocating blocks
on this folio again in the next iteration.

Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
---
 fs/ext4/inode.c | 45 +++++++++++++++++++++++++++++++++++++++------
 1 file changed, 39 insertions(+), 6 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index d0db6e3bf158..b51de58518b2 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -877,20 +877,44 @@ static void ext4_update_bh_state(struct buffer_head *bh, unsigned long flags)
 	} while (unlikely(!try_cmpxchg(&bh->b_state, &old_state, new_state)));
 }
 
+/*
+ * Make sure that the current journal transaction has enough credits to map
+ * one extent. Return -EAGAIN if it cannot extend the current running
+ * transaction.
+ */
+static inline int ext4_journal_ensure_extent_credits(handle_t *handle,
+						     struct inode *inode)
+{
+	int needed_credits;
+	int ret;
+
+	needed_credits = ext4_chunk_trans_blocks(inode, 1);
+	ret = __ext4_journal_ensure_credits(handle, needed_credits,
+					    needed_credits, 0);
+	return ret <= 0 ? ret : -EAGAIN;
+}
+
 static int _ext4_get_block(struct inode *inode, sector_t iblock,
 			   struct buffer_head *bh, int flags)
 {
 	struct ext4_map_blocks map;
+	handle_t *handle = ext4_journal_current_handle();
 	int ret = 0;
 
 	if (ext4_has_inline_data(inode))
 		return -ERANGE;
 
+	/* Make sure transaction has enough credits for this extent */
+	if (flags & EXT4_GET_BLOCKS_CREATE) {
+		ret = ext4_journal_ensure_extent_credits(handle, inode);
+		if (ret)
+			return ret;
+	}
+
 	map.m_lblk = iblock;
 	map.m_len = bh->b_size >> inode->i_blkbits;
 
-	ret = ext4_map_blocks(ext4_journal_current_handle(), inode, &map,
-			      flags);
+	ret = ext4_map_blocks(handle, inode, &map, flags);
 	if (ret > 0) {
 		map_bh(bh, inode->i_sb, map.m_pblk);
 		ext4_update_bh_state(bh, map.m_flags);
@@ -1374,8 +1398,9 @@ static int ext4_write_begin(struct file *file, struct address_space *mapping,
 				ext4_orphan_del(NULL, inode);
 		}
 
-		if (ret == -ENOSPC &&
-		    ext4_should_retry_alloc(inode->i_sb, &retries))
+		if (ret == -EAGAIN ||
+		    (ret == -ENOSPC &&
+		     ext4_should_retry_alloc(inode->i_sb, &retries)))
 			goto retry_journal;
 		folio_put(folio);
 		return ret;
@@ -2324,6 +2349,11 @@ static int mpage_map_one_extent(handle_t *handle, struct mpage_da_data *mpd)
 	int get_blocks_flags;
 	int err, dioread_nolock;
 
+	/* Make sure transaction has enough credits for this extent */
+	err = ext4_journal_ensure_extent_credits(handle, inode);
+	if (err < 0)
+		return err;
+
 	trace_ext4_da_write_pages_extent(inode, map);
 	/*
 	 * Call ext4_map_blocks() to allocate any delayed allocation blocks, or
@@ -2446,7 +2476,7 @@ static int mpage_map_and_submit_extent(handle_t *handle,
 			 * In the case of ENOSPC, if ext4_count_free_blocks()
 			 * is non-zero, a commit should free up blocks.
 			 */
-			if ((err == -ENOMEM) ||
+			if ((err == -ENOMEM) || (err == -EAGAIN) ||
 			    (err == -ENOSPC && ext4_count_free_clusters(sb))) {
 				/*
 				 * We may have already allocated extents for
@@ -2953,6 +2983,8 @@ static int ext4_do_writepages(struct mpage_da_data *mpd)
 			ret = 0;
 			continue;
 		}
+		if (ret == -EAGAIN)
+			ret = 0;
 		/* Fatal error - ENOMEM, EIO... */
 		if (ret)
 			break;
@@ -6722,7 +6754,8 @@ vm_fault_t ext4_page_mkwrite(struct vm_fault *vmf)
 		}
 	}
 	ext4_journal_stop(handle);
-	if (err == -ENOSPC && ext4_should_retry_alloc(inode->i_sb, &retries))
+	if (err == -EAGAIN ||
+	    (err == -ENOSPC && ext4_should_retry_alloc(inode->i_sb, &retries)))
 		goto retry_alloc;
 out_ret:
 	ret = vmf_fs_error(err);
-- 
2.46.1