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    SubjectRe: [PATCH 1/4] compcache: xvmalloc memory allocator
    On 08/25/2009 08:22 PM, Nitin Gupta wrote:
    > On 08/25/2009 03:16 AM, Hugh Dickins wrote:
    >> On Tue, 25 Aug 2009, Nitin Gupta wrote:
    >>> On 08/25/2009 02:09 AM, Hugh Dickins wrote:
    >>>> On Tue, 25 Aug 2009, Nitin Gupta wrote:
    >>>>> On 08/24/2009 11:03 PM, Pekka Enberg wrote:
    >>>>>> What's the purpose of passing PFNs around? There's quite a lot of PFN
    >>>>>> to struct page conversion going on because of it. Wouldn't it make
    >>>>>> more sense to return (and pass) a pointer to struct page instead?
    >>>>> PFNs are 32-bit on all archs
    >>>> Are you sure? If it happens to be so for all machines built today,
    >>>> I think it can easily change tomorrow. We consistently use unsigned
    >>>> long
    >>>> for pfn (there, now I've said that, I bet you'll find somewhere we
    >>>> don't!)
    >>>> x86_64 says MAX_PHYSMEM_BITS 46 and ia64 says MAX_PHYSMEM_BITS 50 and
    >>>> mm/sparse.c says
    >>>> unsigned long max_sparsemem_pfn = 1UL<< (MAX_PHYSMEM_BITS-PAGE_SHIFT);
    >>> For PFN to exceed 32-bit we need to have physical memory> 16TB (2^32
    >>> * 4KB).
    >>> So, maybe I can simply add a check in ramzswap module load to make
    >>> sure that
    >>> RAM is indeed< 16TB and then safely use 32-bit for PFN?
    >> Others know much more about it, but I believe that with sparsemem you
    >> may be handling vast holes in physical memory: so a relatively small
    >> amount of physical memory might in part be mapped with gigantic pfns.
    >> So if you go that route, I think you'd rather have to refuse pages
    >> with oversized pfns (or refuse configurations with any oversized pfns),
    >> than base it upon the quantity of physical memory in the machine.
    >> Seems ugly to me, as it did to Pekka; but I can understand that you're
    >> very much in the business of saving memory, so doubling the size of some
    >> of your tables (I may be oversimplifying) would be repugnant to you.
    >> You could add a CONFIG option, rather like CONFIG_LBDAF, to switch on
    >> u64-sized pfns; but you'd still have to handle what happens when the
    >> pfn is too big to fit in u32 without that option; and if distros always
    >> switch the option on, to accomodate the larger machines, then there may
    >> have been no point to adding it.
    > Thanks for these details.
    > Now I understand that use of 32-bit PFN on 64-bit archs is unsafe. So,
    > there is no option but to include extra bits for PFNs or use struct page.
    > * Solution of ramzswap block device:
    > Use 48 bit PFNs (32 + 8) and have a compile time error to make sure that
    > that MAX_PHYSMEM_BITS is < 48 + PAGE_SHIFT. The ramzswap table can
    > accommodate
    > 48-bits without any increase in table size.

    I went crazy. I meant 40 bits for PFN -- not 48. This 40-bit PFN should be
    sufficient for all archs. For archs where 40 + PAGE_SHIFT < MAX_PHYSMEM_BITS
    ramzswap will just issue a compiler error.


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