linux-ext4 - [Question] ext4: different behavior of fsync when use fast commit

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Date:   Thu, 11 Nov 2021 15:10:10 +0800
From:   尹欣 <yinxin.x@...edance.com>
To:     harshadshirwadkar@...il.com, tytso@....edu
Cc:     linux-ext4@...r.kernel.org
Subject: [Question] ext4: different behavior of fsync when use fast commit

Hi,

Recently, I‘m doing some testing with fast commit feature , and found
there is a slight difference on fsync compared with the normal
journaling scheme.

Here is the example:

-mkdir test/

-create&write test/a.txt

-fsync test/a.txt

-crash (before a full commit)

If fast commit is used then “a.txt” will lost. While the normal
journaling can recover it.

Refer to the description of fsync [1],  fsync will not guarantee the
parent directory to be persisted. So I think it is not an issue.

But fast commit did change the behavior of fsync in ext4,  is this as expected ?

For the root cause of this difference, I found that fast commit will
not add a EXT4_FC_TAG_CREAT tag for directory creation.

In func ext4_fc_commit_dentry_updates(), only directories in s_fc_q
list can be added with EXT4_FC_TAG_CREAT,but seams the newly created
directory inode has no change to be added to s_fc_q.

Shall we just change the “enqueue” param of ext4_fc_track_template()
to 1 , which in __ext4_fc_track_create()?  And make fast commit record
all the inode creation, and do the same things as normal journaling.

[1] https://man7.org/linux/man-pages/man2/fdatasync.2.html

BR,

Xin Yin