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

David Rientjes
2010-08-26T03:09:29Z

On Wed, 25 Aug 2010, Ted Ts'o 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.
>

Ok, and we'll make it a WARN_ON_ONCE() to be nice to the kernel log.
Although the current patchset does this with WARN_ON_ONCE(1, ...) instead,
this serves to ensure that we aren't dependent on the page allocator's
implementation to always loop for order < PAGE_ALLOC_COSTLY_ORDER in which
case the loop in the _nofail() functions would actually do something.

> > 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.
>

On my 8GB machine, /proc/zoneinfo says the min watermark for ZONE_NORMAL
is 5086 pages, or ~20MB. GFP_ATOMIC would allow access to ~12MB of that,
so perhaps we should consider this is an acceptable abuse of GFP_ATOMIC as
a fallback behavior when GFP_NOFS or GFP_NOIO fails?