Red Hat Bugzilla – Bug 44384
mem=XXX uses more memory than stated
Last modified: 2007-04-18 12:33:40 EDT
From Bugzilla Helper: User-Agent: Mozilla/4.77 [en] (X11; U; Linux 2.4.2-2 i686) Description of problem: I am running 7.1 on a i686 box with 256 of memory. When the system is booted normally, a cat /proc/iomem shows: 040ffc00-0fffffff : System RAM which is correct. However, if I set mem=248M and reboot I get: 040ffc00-0f862bff : System RAM instead of 040ffc00 - 0f800000 if I try mem=247M I get 0f0ffc00-0f762bff. It appears that the kernel is reserving 0x62bff bytes of memory more that what it was told it has. I am trying to reserve a large chunk of memory for DMA transfers, and I need to write a mmap method in my driver, so getting this EXACTLY right is important. My workaround was to set mem=247M which freed up about 8.5 MB and just used the top 8M. How reproducible: Always Steps to Reproduce: 1.linux mem=248M 2.cat /proc/iomem 3. Actual Results: 040ffc00-0f862bff : System RAM Expected Results: 040ffc00 - 0f800000 : System RAM Additional info:
If you have a hole in the memory-map it seems the kernel accounts for that ;) 404479 bytes is quite believable as size of such hole (380kb in the lower 1Mb region + bios reserved regions)
Yes, that is exactly what is happening, but that is misleading. If I put mem=247M, I *thought* I am saying that I only have 247M of memory. The kernel is *using* 247M of memory, but because of the hole, it actually uses MORE. {warrenj} cat /proc/iomem 00000000-0009f7ff : System RAM 0009f800-0009ffff : reserved 000a0000-000bffff : Video RAM area 000c0000-000c7fff : Video ROM 000e0000-000effff : Extension ROM 000f0000-000fffff : System ROM 00100000-040fd7ff : System RAM 00100000-002557df : Kernel code 002557e0-0026c80b : Kernel data 040fd800-040ff7ff : ACPI Tables 040ff800-040ffbff : ACPI Non-volatile Storage 040ffc00-0f762bff : System RAM <================================== e4000000-e7ffffff : PCI device 1593:2041 (Bops Inc) e8000000-ebffffff : PCI device 1593:2041 (Bops Inc) ec000000-efffffff : PCI device 1593:2041 (Bops Inc) f0000000-f3ffffff : PCI device 1593:2041 (Bops Inc) f4000000-f4007fff : Yamaha Corporation YMF-740C [DS-1L Audio Controller] f4008000-f4008fff : PCI device 100b:0020 (National Semiconductor Corporation) f4008000-f4008fff : eth0 f5000000-f5ffffff : PCI Bus #01 f5000000-f5ffffff : nVidia Corporation Riva TnT2 [NV5] f8000000-fbffffff : Intel Corporation 440BX/ZX - 82443BX/ZX Host bridge fc000000-fdffffff : PCI Bus #01 fc000000-fdffffff : nVidia Corporation Riva TnT2 [NV5] If you don't put a mem= option, it ends at the memory boundary, so this behaviour is not consistant!
I agree that this isn't consisitent behavior. Itw ill be fixed in the next kernel we release.