linux-kernel - Re: openat(..., AT_UNLINKED) was Re: New copyfile system call

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Date:	Sun, 31 Mar 2013 19:14:15 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Pavel Machek <pavel@....cz>
CC:	"Myklebust, Trond" <Trond.Myklebust@...app.com>,
	Andreas Dilger <adilger@...ger.ca>,
	Jörn Engel <joern@...fs.org>,
	Andy Lutomirski <luto@...capital.net>,
	Zach Brown <zab@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Chris L. Mason" <clmason@...ionio.com>,
	Christoph Hellwig <hch@...radead.org>,
	Alexander Viro <aviro@...hat.com>,
	"Martin K. Petersen" <mkp@....net>, Hannes Reinecke <hare@...e.de>,
	Joel Becker <jlbec@...lplan.org>
Subject: Re: openat(..., AT_UNLINKED) was Re: New copyfile system call - discuss
 before LSF?

On 03/31/2013 06:50 PM, Pavel Machek wrote:
> On Sun 2013-03-31 18:44:53, Myklebust, Trond wrote:
>> On Sun, 2013-03-31 at 20:32 +0200, Pavel Machek wrote:
>>>>>>> Hmm. open_deleted_file() will still need to get a directory... so it
>>>>>>> will still need a path. Perhaps open("/foo/bar/mnt", O_DELETED) would
>>>>>>> be acceptable interface?
>>>>>> ...and what's the big plan to make this work on anything other than ext4 and btrfs?
>>>>> Deleted but open files are from original unix, so it should work on
>>>>> anything unixy (minix, ext, ext2, ...).
>>>> minix, ext, ext2... are not under active development and haven't been
>>>> for more than a decade.
>>>>
>>>> Take a look at how many actively used filesystems out there that have
>>>> some variant of sillyrename(), and explain what you want to do in those
>>>> cases.
>>> Well. Yes, there are non-unix filesystems around. You have to deal
>>> with silly files on them, and this will not be different.
>> So this would be a local POSIX filesystem only solution to a problem
>> that has yet to be formulated?
> Problem is "clasical create temp file then delete it" is racy. See the
> archives. That is useful & common operation.

Which race are you concerned with exactly?

User wants to test for a file with name "foo.txt"

* create "foo.txt~" (or whatever)
* write contents into "foo.txt~"
* rename "foo.txt~" to "foo.txt"

Until rename is done, the file does not exists and is not complete. You will 
potentially have a garbage file to clean up if the program (or system) crashes, 
but that is not racy in a classic sense, right?

This is more of a garbage clean up issue?

Regards,

Ric

>
> Problem is "atomicaly create file at target location with guaranteed
> right content". That's also in the archives. Looks useful if someone
> does rsync from your directory.
>
> Non-POSIX filesystems have problems handling deleted files, but that
> was always the case. That's one of the reasons they are seldomly used
> for root filesystems.
>
> 									Pavel

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