Date:	Wed, 29 Dec 2010 13:42:01 +0100
From:	Olaf van der Spek <olafvdspek@...il.com>
To:	Amir Goldstein <amir73il@...il.com>
Cc:	Ric Wheeler <rwheeler@...hat.com>, "Ted Ts'o" <tytso@....edu>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-ext4@...r.kernel.org
Subject: Re: Atomic file data replace API

On Wed, Dec 29, 2010 at 10:20 AM, Amir Goldstein <amir73il@...il.com> wrote:
> On Wed, Dec 29, 2010 at 12:58 AM, Olaf van der Spek
> <olafvdspek@...il.com> wrote:
>> On Tue, Dec 28, 2010 at 11:36 PM, Ric Wheeler <rwheeler@...hat.com> wrote:
>>> I think that various developers have answered this for you several times.
>>
>> Not really, unfortunately. Haven't seen a single link to code that
>> shows how to do it properly.
>> Temp file, fsync, rename is often mentioned but that skips the
>> preserving meta-data part and this part, which you also skipped:
>> One use case would be updating a file in a safe way when you have
>> write access to that file but not to anything else.
>>
>
> I think it is safe to say that the *only* option you have now is "temp
> file, fsync, rename".

I'm really looking for a concrete code snippet/function that does this.
For example, file permissions should definitely be preserved.

> There is no "generic atomic file data replace API in Linux", though it
> is available via
> private ioctl for XFS and EXT4.
>
> You have started a bit of a storm with your previous thread, which
> doesn't help you
> much in moving forward in the current thread (previous thread is still
> more popular).
> I suggest that you humbly swallow you need to know WHY is it hard to implement
> non-durable atomic API and focus your attention on the very achievable
> data replace API.
>
> IMHO, implementing atomic swap_inodes_data operation shouldn't be difficult
> in most file systems (only implementation is simple, but testing and
> maintaining
> is not to be taken lightly).
> Something along the lines of:
> 1. aquire inodes write/truncate locks
> 2. start transaction
> 3. check/update quota limits
> 4. swap inodes i_data content
> 5. invalidate (or swap?) inodes page caches
> 6. mark inodes dirty
> 7. end transaction & release locks
>
> The real challenge would be to get everyone to agree on a common API
> and carve it in stone to the kernel's ABI (is it just swap_inodes_data?
> maybe also swap_inode_data_ranges? what about some options?)

Swapping data is an improvement but still not ideal. The API is also
more complex than O_ATOMIC.

> Also, as wacky and (some say) faulty the UNIX permissions models is,
> current systems have grown old with it, and even 'improving' the behavior
> of some applications, may wake up sleeping monsters, so it will not
> be done until enough people have pointed out security or usability
> issues, which could not be solved otherwise.

Each app makes it's own decision about what API to use. Supporting
atomic stuff doesn't change the behaviour of existing apps.

> In other words, until you find an *application* that wants to allow other
> user to modify the content of a file and preserve it's metadata and ownership.
> And unless that application cannot find a better way to achieve what it wanted
> to do in the first place, or unless that application already has a
> large install base
> which suffers from *a problem*, you will not have proven *the need*.

Maybe I should ask devs of some large apps on their take of this issue.

> Maybe preserving privileged extended attributes is *a need*.
> I wouldn't know myself.

Olaf
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