On Thu, Jan 24, 2013 at 09:45:50AM -0000, David Laight wrote: > > + 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 > > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
