Messages in this thread | | | Date | Wed, 26 Aug 2009 00:33:40 +0530 | From | Nitin Gupta <> | Subject | Re: [PATCH 1/4] compcache: xvmalloc memory allocator |
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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.
Thanks, Nitin
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