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

[Ted Ts'o <tytso@mit.edu>]

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