linux-ext4 - Re: infinite getdents64 loop

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Date:	Tue, 31 May 2011 11:26:30 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	Ted Ts'o <tytso@....edu>
Cc:	Bernd Schubert <bernd.schubert@...m.fraunhofer.de>,
	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 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.

>> 3) Disable dirindexing for readdirs
> 
> That won't work, since it will break POSIX compliance.  Once again,
> we're tied by the decisions made decades ago...


Cheers, Andreas





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