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

On 01/09/2012 02:51 PM, Seth Jennings wrote:

quoted

+	area = &get_cpu_var(zs_map_area);
+	if (off + class->size <= PAGE_SIZE) {
+		/* this object is contained entirely within a page */
+		area->vm_addr = kmap_atomic(page);
+	} else {
+		/* this object spans two pages */
+		struct page *nextp;
+
+		nextp = get_next_page(page);
+		BUG_ON(!nextp);
+
+
+		set_pte(area->vm_ptes[0], mk_pte(page, PAGE_KERNEL));
+		set_pte(area->vm_ptes[1], mk_pte(nextp, PAGE_KERNEL));
+
+		/* We pre-allocated VM area so mapping can never fail */
+		area->vm_addr = area->vm->addr;
+	}

This bit appears to be trying to make kmap_atomic() variant that can map
two pages in to contigious virtual addresses.  Instead of open-coding it
in a non-portable way like this, should we just make a new kmap_atomic()
variant that does this?

From the way it's implemented, I _think_ you're guaranteed to get two
contiguous addresses if you do two adjacent kmap_atomics() on the same CPU:

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.

--
Seth

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>