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    SubjectRe: [PATCH 1/5] staging: zsmalloc: zsmalloc memory allocation library
    On 02/08/2012 11:39 AM, Dave Hansen wrote:

    > On 02/06/2012 09:26 AM, Seth Jennings wrote:
    >> On 01/26/2012 01:12 PM, Dave Hansen wrote:
    >>> 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).


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