Messages in this thread Patch in this message | | | Date | Tue, 25 Aug 2009 20:22:21 +0530 | From | Nitin Gupta <> | Subject | Re: [PATCH 1/4] compcache: xvmalloc memory allocator |
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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.
--- ramzswap_new.h 2009-08-25 20:10:38.054033804 +0530 +++ ramzswap.h 2009-08-25 20:09:28.386069100 +0530 @@ -110,9 +110,9 @@
/* Indexed by page no. */ struct table { - u32 pagenum_1; + u32 pagenum; u16 offset; - u8 pagenum_2; + u8 count; /* object ref count (not yet used) */ u8 flags; };
(removal for 'count' field will hurt later when we implement memory defragmentation support).
* Solution for allocator:
Use struct page instead of PFN. This is better than always using 64-bit PFNs since we get rid of all casts. Use of 48-bit PFNs as above will create too much mess. However, use of struct page increases per-pool overhead by 4K on 64-bit systems. This should be okay.
Please let me know if you have any comments. I will make these changes in next revision.
There is still some problem with memory allocator naming. Its no longer a separate module, the symbols are not exported and its now compiled with ramzswap block driver itself. So, I am hoping xv_malloc() does not causes any confusion with any existing name now. It really should not cause any confustion. I would love to retain this name for allocator.
Thanks, Nitin
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