How kernel determind the low and high memory zone?
2009 March 24
I thought Linux fixed highmem zone and low mem zone at the compile time. But, I was wrong. I even found the command line parameter named ‘highmem=’ which can set the highmem size.
Actually, this calculation is done during the initialization and following is the code which actually do the thing.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 | /* * Determine low and high memory ranges: */ unsigned long __init find_max_low_pfn(void) { unsigned long max_low_pfn; max_low_pfn = max_pfn; if (max_low_pfn > MAXMEM_PFN) { if (highmem_pages == -1) highmem_pages = max_pfn - MAXMEM_PFN; if (highmem_pages + MAXMEM_PFN < max_pfn) max_pfn = MAXMEM_PFN + highmem_pages; if (highmem_pages + MAXMEM_PFN > max_pfn) { printk("only %luMB highmem pages available, ignoring highmem size of %uMB.\n", pages_to_mb(max_pfn - MAXMEM_PFN), pages_to_mb(highmem_pages)); highmem_pages = 0; } max_low_pfn = MAXMEM_PFN; #ifndef CONFIG_HIGHMEM /* Maximum memory usable is what is directly addressable */ printk(KERN_WARNING "Warning only %ldMB will be used.\n", MAXMEM>>20); if (max_pfn > MAX_NONPAE_PFN) printk(KERN_WARNING "Use a PAE enabled kernel.\n"); else printk(KERN_WARNING "Use a HIGHMEM enabled kernel.\n"); max_pfn = MAXMEM_PFN; #else /* !CONFIG_HIGHMEM */ #ifndef CONFIG_X86_PAE if (max_pfn > MAX_NONPAE_PFN) { max_pfn = MAX_NONPAE_PFN; printk(KERN_WARNING "Warning only 4GB will be used.\n"); printk(KERN_WARNING "Use a PAE enabled kernel.\n"); } #endif /* !CONFIG_X86_PAE */ #endif /* !CONFIG_HIGHMEM */ } else { if (highmem_pages == -1) highmem_pages = 0; #ifdef CONFIG_HIGHMEM if (highmem_pages >= max_pfn) { printk(KERN_ERR "highmem size specified (%uMB) is bigger than pages available (%luMB)!.\n", pages_to_mb(highmem_pages), pages_to_mb(max_pfn)); highmem_pages = 0; } if (highmem_pages) { if (max_low_pfn-highmem_pages < 64*1024*1024/PAGE_SIZE){ printk(KERN_ERR "highmem size %uMB results in smaller than 64MB lowmem, ignoring it.\n", pages_to_mb(highmem_pages)); highmem_pages = 0; } max_low_pfn -= highmem_pages; } #else if (highmem_pages) printk(KERN_ERR "ignoring highmem size on non-highmem kernel!\n"); #endif } return max_low_pfn; } |
No comments yet