linux-kernel - Re: [git pull] vfs pile 1

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Date:	Thu, 30 Jan 2014 11:02:49 +0900
From:	Kim Jaegeuk <jaegeuk.kim@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@....de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Jaegeuk Kim <jaegeuk.kim@...sung.com>
Subject: Re: [git pull] vfs pile 1

2014-01-29 Jan Kara <jack@...e.cz>:
> On Tue 28-01-14 19:26:08, Linus Torvalds wrote:
>> On Mon, Jan 27, 2014 at 6:25 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
>> >         Assorted stuff; the biggest pile here is Christoph's ACL series.
>> > Plus assorted cleanups and fixes all over the place...  There will be
>> > another pile later this week.
>>
>> The posix_acl_chmod() code looks wrong.
>>
>> Not that it looked right before either, but whatever. The code
>> basically looks like some variation of this in most setattr()
>> implementations:
>>
>>         if (ia_valid & ATTR_MODE)
>>                 rc = posix_acl_chmod(inode, inode->i_mode);
>>
>> but the mode we're changing to (and what ATTR_MODE guards) is actually
>> attr->ia_mode, not inode->i_mode.
>   Yes, but posix_acl_chmod() is called after setattr_copy() was done so
> inode->i_mode should be the same as attr->ia_mode. Whether i_mode or
> ia_mode is mode logical depends on whether you view posix_acl_chmod() as
> "sync current i_mode into acls" or "reflect this i_mode change in acls".
> I agree the function name suggests more the latter semantics.
>
>> And quite frankly, passing in inode->i_mode looks stupid, since we're
>> already passing in the inode pointer, so that's just redundant and
>> pointless information.
>   Yes, it looks stupid. We could almost drop that argument, except that f2fs
> tries to play some tricks with i_mode and stores i_mode in a different
> place when acls are enabled. Huh? Jaegeuk, can you explain why are you
> doing that?

As described to Christoph before, the reason is for acl consistency
between on-disk xattr->mode and on-disk inode->mode.

Previously, there are three i_modes managed by:
                       inode->mode   on-disk xattr->mode  on-disk->i_mode
f2fs_setattr            [x]                       y                       y
[update_inode]       x                        y                      [x]
[checkpoint]            x                       [y]                      x
__f2fs_setxattr        x                       [x]                      x

In this flow, f2fs is able to break the consistency between on-disk
xattr->mode and on-disk->i_mode after checkpoint followed by
sudden-power-off.

So, fi->i_mode was introduced to address the problem.
The new f2fs_setattr triggers:
                inode->mode   fi->i_mode  on-disk xattr->mode  on-disk->i_mode
f2fs_setattr             y                [x]                     y
                      y
[update_inode]       y                 x                      y
                  y
[checkpoint]            y                 x                      y
                     y
__f2fs_setxattr       [x]                x                     [x]
                   [x]

Finally, __f2fs_setxattr synchronizes inode->mode, on-disk xattr->mode,
and on-disk inode->i_mode all together.

Am I missing something?

Thanks,

>
>                                                                 Honza
> --
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
> --
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