Commit 154922b3 authored by Zhang Yi's avatar Zhang Yi Committed by Theodore Ts'o
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ext4: don't order data when zeroing unwritten or delayed block 



When zeroing out a written partial block, it is necessary to order the
data to prevent exposing stale data on disk. However, if the buffer is
unwritten or delayed, it is not allocated as written, so ordering the
data is not required. This can prevent strange and unnecessary ordered
writes when appending data across a region within a block.

Assume we have a 2K unwritten file on a filesystem with 4K blocksize,
and buffered write from 3K to 4K. Before this patch,
__ext4_block_zero_page_range() would add the range [2k,3k) to the
ordered range, and then the JBD2 commit process would write back this
block. However, it does nothing since the block is not mapped as
written, this folio will be redirtied and written back agian through the
normal write back process.

Signed-off-by: default avatarZhang Yi <yi.zhang@huawei.com>
Reviewed-by: default avatarJan Kara <jack@suse.cz>
Reviewed-by: default avatarBaokun Li <libaokun1@huawei.com>
Link: https://patch.msgid.link/20251223011927.34042-1-yi.zhang@huaweicloud.com


Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
parent 26f260ce

fs/ext4/inode.c

+6 −2
Original line number Diff line number Diff line
@@ -4133,9 +4133,13 @@ static int __ext4_block_zero_page_range(handle_t *handle, 
	if (ext4_should_journal_data(inode)) {

		err = ext4_dirty_journalled_data(handle, bh);

	} else {

		err = 0;

		mark_buffer_dirty(bh);

		if (ext4_should_order_data(inode))

		/*

		 * Only the written block requires ordered data to prevent

		 * exposing stale data.

		 */

		if (!buffer_unwritten(bh) && !buffer_delay(bh) &&

		    ext4_should_order_data(inode))

			err = ext4_jbd2_inode_add_write(handle, inode, from,

					length);

	}