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

Ted Ts'o
2010-08-26T01:48:47Z

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