linux-ext4 - Re: [patch 1/5] mm: add nofail variants of kmalloc kcalloc and kzalloc

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Date:	Wed, 25 Aug 2010 21:48:47 -0400
From:	Ted Ts'o <tytso@....edu>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Jens Axboe <jaxboe@...ionio.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Neil Brown <neilb@...e.de>, Alasdair G Kergon <agk@...hat.com>,
	Chris Mason <chris.mason@...cle.com>,
	Steven Whitehouse <swhiteho@...hat.com>,
	Jan Kara <jack@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>,
	"linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
	"cluster-devel@...hat.com" <cluster-devel@...hat.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"reiserfs-devel@...r.kernel.org" <reiserfs-devel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [patch 1/5] mm: add nofail variants of kmalloc kcalloc and
 kzalloc

On Wed, Aug 25, 2010 at 05:30:42PM -0700, David Rientjes wrote:
> 
> We certainly hope that nobody will reimplement the same function without 
> the __deprecated warning, especially for order < PAGE_ALLOC_COSTLY_ORDER 
> where there's no looping at a higher level.  So perhaps the best 
> alternative is to implement the same _nofail() functions but do a 
> WARN_ON(get_order(size) > PAGE_ALLOC_COSTLY_ORDER) instead?

Yeah, that sounds better.

> I think it's really sad that the caller can't know what the upper bounds 
> of its memory requirement are ahead of time or at least be able to 
> implement a memory freeing function when kmalloc() returns NULL.

Oh, we can determine an upper bound.  You might just not like it.
Actually ext3/ext4 shouldn't be as bad as XFS, which Dave estimated to
be around 400k for a transaction.  My guess is that the worst case for
ext3/ext4 is probably around 256k or so; like XFS, most of the time,
it would be a lot less.  (At least, if data != journalled; if we are
doing data journalling and every single data block begins with
0xc03b3998U, we'll need to allocate a 4k page for every single data
block written.)  We could dynamically calculate an upper bound if we
had to.  Of course, if ext3/ext4 is attached to a network block
device, then it could get a lot worse than 256k, of course.

	      	      	      	    	- Ted

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