linux-kernel - Re: [PATCH 1/3]fs/inode: iunique() Optimize Performance

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Date:	Wed, 25 Nov 2009 12:52:57 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	J?r?my Cochoy <jeremy.cochoy@...il.com>
Cc:	Liuweni <qingshenlwy@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	strongzgy <strongzgy@...il.com>, xgr178 <xgr178@....com>,
	Liu Hui <onlyflyer@...il.com>, viro <viro@...iv.linux.org.uk>,
	akpm <akpm@...ux-foundation.org>, jack <jack@...e.cz>,
	npiggin <npiggin@...e.de>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 1/3]fs/inode: iunique() Optimize Performance

On Wed, Nov 25, 2009 at 08:20:02PM +0100, J?r?my Cochoy wrote:
> Hello,
> 
> There is something strange in iunique : what will happend if all inode
> between max_reserved+1 and (unsinged in)(0-1) ? Will it make an
> infinite loop or an interruption can happen and make an inode become
> free?

Another process can free an inode while this loop is executing.

> In that case, it will be better to stop search when counter overflow, no?

If you have all four billion inodes allocated, you have significantly
bigger problems than this loop.  For a start, at 600 bytes per inode,
I'd like to see your machine with 2.4TB of memory.  Then there's the
size of your inode hash, and the depth of the chains within it.

> Will it not be better to use a field max_ino_used (in superblock, for
> exemple) where we store the last inode allocated with iunique and make
> a search only if max_ino_used become to (unsigned)(-1) ?
> 
> But, if iunique is here to provide a solution in order to generate
> unused inode in filesystem which have various inode number, it's
> better to use a list of used ino, in a short hash table which use the
> first 8 bits of the inode, always use the same function to create a
> new inode and look at the head if we can add a new inode with bigger
> ino and still in the range. (But i think filesystems developper prefer
> to write ther own functions in order to do that, no?)
> 
> Well, if we want to stop in case of full inode filesystem, we can put
> the first condition in the head and add change return as :
> return inode->i_ino > max_reserved ? res : 0; // 0 might "i can't find
> an inode after max_reserved"

Gloves.

http://thedailywtf.com/Articles/The_Complicator_0x27_s_Gloves.aspx

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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