linux-ext4 - Re: [PATCH v2] Check for immutable flag in fallocate path

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Message-ID: <4D70D89C.6090307@gmail.com>
Date:	Fri, 04 Mar 2011 13:18:36 +0100
From:	Marco Stornelli <marco.stornelli@...il.com>
To:	Dave Chinner <david@...morbit.com>
CC:	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-ext4@...r.kernel.org, linux-btrfs@...r.kernel.org,
	cluster-devel@...hat.com, xfs@....sgi.com,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>, tytso@....edu
Subject: Re: [PATCH v2] Check for immutable flag in fallocate path

Il 04/03/2011 09:17, Marco Stornelli ha scritto:
> Hi Dave,
> 
> Il 03/03/2011 22:39, Dave Chinner ha scritto:
>> WTF?  Why does append mode have any effect on whether we can punch
>> holes in a file or not? There's no justification for adding this in
>> the commit message. Why is it even in a patch that is for checking
>> immutable inodes? What is the point of adding it, when all that will
>> happen is people will switch to XFS_IOC_UNRESVSP which has never had
>> this limitation?
> 
> So according to you, it's legal to do an "unreserve" operation on an
> append-only file. It's not the same for me, but if the community said
> that this is the right behavior then ok.
> 
>>
>> And this asks bigger questions - why would you allow preallocate
>> anywhere but at or beyond EOF on an append mode inode? You can only
>> append to the file, so if you're going to add limitations based on
>> the append flag, you need to think this through a bit more....
>>
> 
> I don't understand this point. The theory of operation was:
> 
> 1) we don't allow any operation (reserve/unreserve) on a immutable file;
> 2) we don't allow *unreserve* operation on an append-only file (this
> check makes sense only for fs that support the unreserve operation).
> 
>>
>> Also, like Christoph said, these checks belong in the generic code,
>> not in every filesystem. The same checks have to be made for every
>> filesystem, so they should be done before calling out the
>> filesystems regardless of what functionality the filesystem actually
>> supports.
>>
> 
> This was related to the first point, if we remove it then it's ok to
> check in a common code. Even if I think we should do the check under the
> inode lock to avoid race between fallocate and setattr, isn't it?
> 

Oops, I meant setflags in ioctl path, sorry. At this point I'm waiting
for response about how to manage the append flag and how to manage the
lock on the flags. Ted pointed out that a proper fix would be to avoid
the lock and use bit operation but it requires a deep modification on
several fs and it could be a separate patch and code review, so I think
we can choice to use lock/unlock in do_fallocate. I'll resend the patch.

Marco
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