From: "Theodore Y. Ts'o" <tytso@mit.edu>
To: xiaohui li <lixiaohui1@xiaomi.corp-partner.google.com>
Cc: "Xiaohui1 Li 李晓辉" <lixiaohui1@xiaomi.com>,
"linux-ext4@vger.kernel.org" <linux-ext4@vger.kernel.org>,
"harshadshirwadkar@gmail.com" <harshadshirwadkar@gmail.com>
Subject: Re: 答复: [External Mail]Re: [PATCH v3 09/13] ext4: fast-commit commit path changes
Date: Sun, 3 Nov 2019 22:22:12 -0500 [thread overview]
Message-ID: <20191104032212.GA12046@mit.edu> (raw)
In-Reply-To: <CAAJeciVYOAWzsjAtL7SNmpFQH60z0MB53OPE3hZ==_oBB0N3dQ@mail.gmail.com>
On Mon, Nov 04, 2019 at 09:01:28AM +0800, xiaohui li wrote:
>
> when in writeback mode, inode' data has not to be waited in jbd2
> thread, so the fsync time cost is also reduced.
> meawhile, writeback mode also can guarantee filesystem consistency in
> os crash-reboot conditions,
> with only one drawback is that it will cause security problems such as
> stale data will be seen.
It's not just stale data; in data=writeback, today if a file gets
deleted, its blocks are immediately eligible to be reused. If there
is a crash before the transaction is committed, there could be a file
that would have deleted (and perhaps replaced) that doesn't in fact
get deleted, but its data blocks will have been corrupted.
I'm not fond of that particular behavior, and I may look to fix it,
but in general, data=writeback means that data blocks may be corrupted
or contain stale data after a crash --- for blocks that were freshly
created, or for a file that might have been deleted, but except for
the crash which means that the file deletion doesn't actually get
corrupted.
> but in android system with file encryption enabled, there is no
> security problem as files are all encryped.
> but user will see wrong file data in system crash-reboot conditions
> with writeback mode enabled.
If all files are encrypted, then yes, the chances of stale data
causing security issues is significantly reduced.
But see also the case of a file which is deleted immediately before a
crash. Things are more complex in terms of the data gauarantees after
a crash, which is why data=ordered is the default.
Regards,
- Ted
next prev parent reply other threads:[~2019-11-04 3:22 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <1571900042725.99617@xiaomi.com>
2019-10-24 20:18 ` [PATCH v3 09/13] ext4: fast-commit commit path changes Theodore Y. Ts'o
[not found] ` <1572349386604.43878@xiaomi.com>
2019-10-29 21:35 ` Theodore Y. Ts'o
2019-10-30 4:28 ` 答复: [External Mail]Re: " Xiaohui1 Li 李晓辉
2019-10-30 14:26 ` Theodore Y. Ts'o
2019-11-04 1:01 ` xiaohui li
2019-11-04 3:22 ` Theodore Y. Ts'o [this message]
2019-11-08 7:58 ` xiaohui li
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