linux-ext4 - Re: [PATCH 5 2/4] Return 32/64-bit dir name hash according to usage type

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Date:	Tue, 06 Mar 2012 10:59:55 +0100
From:	Bernd Schubert <bernd.schubert@...m.fraunhofer.de>
To:	"Ted Ts'o" <tytso@....edu>
CC:	linux-nfs@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, Fan Yong <yong.fan@...mcloud.com>,
	bfields@...hat.com, sandeen@...hat.com,
	Andreas Dilger <adilger@...mcloud.com>
Subject: Re: [PATCH 5 2/4] Return 32/64-bit dir name hash according to usage
 type

On 03/06/2012 03:28 AM, Ted Ts'o wrote:
> On Tue, Mar 06, 2012 at 01:40:05AM +0100, Bernd Schubert wrote:
>>
>> Yeah, you are right, we also should check for 64-bit EOF. But
>> wouldn't be something like this be better?
>>
>> 	/* check for hash collision */
>> 	if(is_32bit_api() ) {
>> 		if (hash == (EXT4_HTREE_EOF_32BIT<<   1))
>> 			hash = (EXT4_HTREE_EOF_32BIT - 1)<<   1;
>> 	} else {
>> 		if (hash == (EXT4_HTREE_EOF_64BIT<<   1))
>> 			hash = (EXT4_HTREE_EOF_64BIT - 1)<<   1;
>> 	}
>
> Actually, neither change is needed, now that I look at things more
> closely.  hash is a __u32, so it could never been
> EXT4_HTREE_EOF_64BIT.  But given that we won't let major hash become
> larger than 0xfffffffc, that means the largest possible position value
> is 0x7ffffffeffffffff.  So using an EOF value of 0x0x7fffffffffffffff
> will work fine.

Ah, I looked after 1 a.m., seems that was too late for me to notice.

>
> The bigger problem that I found when I looked more closely at the
> patch is that the patch uses f_flags in places where f_mode needs to
> be used:
>
> static inline loff_t hash2pos(struct file *filp, __u32 major, __u32 minor)
> {
> 	if ((filp->f_flags&  FMODE_32BITHASH) ||
>                     ^^^^^^^
> 	    (!(filp->f_flags&  FMODE_64BITHASH)&&  is_32bit_api()))
>                       ^^^^^^^
> 		return major>>  1;
> 	else
> 		return ((__u64)(major>>  1)<<  32) | (__u64)minor;
> }
>
> static inline __u32 pos2maj_hash(struct file *filp, loff_t pos)
> {
> 	if ((filp->f_flags&  FMODE_32BITHASH) ||
>                     ^^^^^^
> 	    (!(filp->f_mode&  FMODE_64BITHASH)&&  is_32bit_api()))
>                       ^^^^^^
> 		return (pos<<  1)&  0xffffffff;
> 	else
> 		return ((pos>>  32)<<  1)&  0xffffffff;
> }
>
> Which makes me wonder how much this has been tested?

Arg, my bad, I introduced this issue when I converted from f_flags to 
f_mode, seems I forgot all of those above :(
Hrm, I thought I had tested sufficiently, but obviously I did not :( 
Here's the test tool.
http://www.pci.uni-heidelberg.de/tc/usr/bernd/downloads/test_seekdir/

While quickly looking, I think it only affects NFSv2, which I think I 
indeed didn't test. I only run tests for 32 bit and 64-bit user space 
and NFSv3. But yes, NFSv2 is an important test too. Not sure if I will 
find time for that today.

Will send an updated version later on.


Thanks for your review,
Bernd
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