On 02/08/2012 11:39 AM, Dave Hansen wrote:

On 02/06/2012 09:26 AM, Seth Jennings wrote:

quoted

On 01/26/2012 01:12 PM, Dave Hansen wrote:

quoted

void *kmap_atomic_prot(struct page *page, pgprot_t prot)
{
...
        type = kmap_atomic_idx_push();
        idx = type + KM_TYPE_NR*smp_processor_id();
        vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);

I think if you do a get_cpu()/put_cpu() or just a preempt_disable()
across the operations you'll be guaranteed to get two contiguous addresses.

I'm not quite following here.  kmap_atomic() only does this for highmem pages.
For normal pages (all pages for 64-bit), it doesn't do any mapping at all.  It
just returns the virtual address of the page since it is in the kernel's address
space.

For this design, the pages _must_ be mapped, even if the pages are directly
reachable in the address space, because they must be virtually contiguous.

I guess you could use vmap() for that.  It's just going to be slower
than kmap_atomic().  I'm really not sure it's worth all the trouble to
avoid order-1 allocations, though.

vmap() is not just slower but also does memory allocations at various
places. Under memory pressure, this may cause failure in reading a
stored object just because we failed to map it. Also, it allocates VA
region each time its called which is a real big waste when we can simply
pre-allocate 2 * PAGE_SIZE'ed VA regions (per-cpu).

Thanks,
Nitin

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