Re: [PATCH] vhost-net: fall back to vmalloc if high-order allocation fails

On Thu, Jan 24, 2013 at 09:45:50AM -0000, David Laight wrote:

quoted

+	n = kmalloc(sizeof *n, GFP_KERNEL | __GFP_NOWARN);
+	if (!n)
+		n = vmalloc(sizeof *n);

I'm slightly confused by this construct.
I thought kmalloc(... GFP_KERNEL) would sleep waiting for
memory (rather than return NULL).

I realise that (for multi-page sizes) that kmalloc() and
vmalloc() both need to find a contiguous block of kernel
virtual addresses - in different address ranges, and
that vmalloc() then has to find physical memory pages
(which will not be contiguous).

I think this means that kmalloc() is likely to be faster
(if it doesn't have to sleep), but that vmalloc() is
allocating from a much larger resource.

This make me that that large allocations that are not
temporary should probably be allocated with vmalloc().

vmalloc has some issues for example afaik it's not backed by
a huge page so  I think its use puts more stress on the TLB cache.

Is there a 'NO_SLEEP' flag to kmalloc()? is that all
GFP_ATOMIC requests? If so you might try a non-sleeping
kmalloc() with a vmalloc() if it fails.

This all looks as though there should be a GFP_NONCONTIG
flag (or similar) so that kmalloc() can make a decision itself.

Of at least a wrapper - like the one for free().

	David