linux-ext4 - Re: infinite getdents64 loop

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Message-ID: <4DE528DE.5020908@itwm.fraunhofer.de>
Date:	Tue, 31 May 2011 19:43:58 +0200
From:	Bernd Schubert <bernd.schubert@...m.fraunhofer.de>
To:	Andreas Dilger <adilger@...ger.ca>
CC:	"Ted Ts'o" <tytso@....edu>, linux-nfs@...r.kernel.org,
	"linux-ext4@...r.kernel.org List" <linux-ext4@...r.kernel.org>,
	Fan Yong <yong.fan@...mcloud.com>
Subject: Re: infinite getdents64 loop

On 05/31/2011 07:26 PM, Andreas Dilger wrote:
> On 2011-05-31, at 6:35 AM, Ted Ts'o wrote:
>> On Tue, May 31, 2011 at 12:18:11PM +0200, Bernd Schubert wrote:
>>>
>>> Out of interest, did anyone ever benchmark if dirindex provides any
>>> advantages to readdir?  And did those benchmarks include the
>>> disadvantages of the present implementation (non-linear inode
>>> numbers from readdir, so disk seeks on stat() (e.g. from 'ls -l') or
>>> 'rm -fr $dir')?
>>
>> The problem is that seekdir/telldir is terminally broken (and so is
>> NFSv2 for using a such a tiny cookie) in that it fundamentally assumes
>> a linear data structure.  If you're going to use any kind of
>> tree-based data structure, a 32-bit "offset" for seekdir/telldir just
>> doesn't cut it.  We actually play games where we memoize the low
>> 32-bits of the hash and keep track of which cookies we hand out via
>> seekdir/telldir so that things mostly work --- except for NFSv2, where
>> with the 32-bit cookie, you're just hosed.
>>
>> The reason why we have to iterate over the directory in hash tree
>> order is because if we have a leaf node split, half the directories
>> entries get copied to another directory entry, given the promises made
>> by seekdir() and telldir() about directory entries appearing exactly
>> once during a readdir() stream, even if you hold the fd open for weeks
>> or days, mean that you really have to iterate over things in hash
>> order.
>>
>> I'd have to look, since it's been too many years, but as I recall the
>> problem was that there is a common path for NFSv2 and NFSv3/v4, so we
>> don't know whether we can hand back a 32-bit cookie or a 64-bit
>> cookie, so we're always handing the NFS server a 32-bit "offset", even
>> though ew could do better.  Actually, if we had an interface where we
>> could give you a 128-bit "offset" into the directory, we could
>> probably eliminate the duplicate cookie problem entirely.  We just
>> send 64-bits worth of hash, plus the first two bytes of the of file
>> name.
>
> If it's of interest, we've implemented a 64-bit hash mode for ext4 to
> solve just this problem for Lustre.  The llseek() code will return a
> 64-bit hash value on 64-bit systems, unless it is running for some
> process that needs a 32-bit hash value (only NFSv2, AFAIK).
>
> The attached patch can at least form the basis for being able to return
> 64-bit hash values for userspace/NFSv3/v4 when usable.  The patch
> is NOT usable as it stands now, since I've had to modify it from the
> version that we are currently using for Lustre (this version hasn't
> actually been compiled), but it at least shows the outline of what needs
> to be done to get this working.  None of the NFS side is implemented.

Thanks Andreas! I haven't tested it yet, but the generic idea looks 
good. I guess the lower part of the patch (netfilter stuff) got 
accidentally in?


Cheers,
Bernd
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