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From:   =?UTF-8?B?5bC55qyj?= <yinxin.x@bytedance.com>
Date:   Thu, 11 Nov 2021 15:10:10 +0800
Message-ID: <CAK896s5szH8qbDGBDo74hyz5iM4QrjbMJkMjh-f1sZh6K0ozTg@mail.gmail.com>
Subject: [Question] ext4: different behavior of fsync when use fast commit
To:     harshadshirwadkar@gmail.com, tytso@mit.edu
Cc:     linux-ext4@vger.kernel.org
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Hi,


Recently, I=E2=80=98m doing some testing with fast commit feature , and fou=
nd
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 =E2=80=9Ca.txt=E2=80=9D will lost. While the no=
rmal
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 expec=
ted ?


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 =E2=80=9Cenqueue=E2=80=9D param of ext4_fc_track_t=
emplate()
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