linux-kernel - [RFC PATCH v3 13/26] ext4: use reserved metadata blocks when splitting extent in endio

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Date: Sat, 27 Jan 2024 09:58:12 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: linux-ext4@...r.kernel.org
Cc: linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	tytso@....edu,
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	yi.zhang@...wei.com,
	yi.zhang@...weicloud.com,
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	wangkefeng.wang@...wei.com
Subject: [RFC PATCH v3 13/26] ext4: use reserved metadata blocks when splitting extent in endio

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

Now ext4 only reserved space for delalloc for data blocks in
ext4_da_reserve_space(), not reserve space for meta blocks when
allocating data blocks. Besides, if we enable dioread_nolock mount
option, it also not reserve meta blocks for the unwritten extents to
written extents conversion. In order to prevent data loss due to failed
to allocate meta blocks when writing data back, we reserve 2% space or
4096 blocks for meta data, and use EXT4_GET_BLOCKS_PRE_IO to do the
potential split in advance. But all these two methods are just best
efforts, if it's really running out of sapce, there is no difference
between splitting extent when writing data back and when I/Os are
completed, both will result in data loss.

The best way is to reserve enough space for metadata. Before that, we
can at least make sure that things will not get worse if we postpone
splitting extent in endio. Plus, after converting regular file's
buffered write path from buffer_head to iomap, splitting extents in
endio becomes mandatory. Now, in preparation for the future conversion,
let's use reserved sapce in endio too.

Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
---
 fs/ext4/extents.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 6b64319a7df8..48d6d125ec37 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -3722,7 +3722,8 @@ static int ext4_convert_unwritten_extents_endio(handle_t *handle,
 			     (unsigned long long)map->m_lblk, map->m_len);
 #endif
 		err = ext4_split_convert_extents(handle, inode, map, ppath,
-						 EXT4_GET_BLOCKS_CONVERT);
+					EXT4_GET_BLOCKS_CONVERT |
+					EXT4_GET_BLOCKS_METADATA_NOFAIL);
 		if (err < 0)
 			return err;
 		path = ext4_find_extent(inode, map->m_lblk, ppath, 0);
-- 
2.39.2