On 09/01/15 17:46, Mason wrote: > On 09/01/2015 14:13, Russell King - ARM Linux wrote: > >> On Fri, Jan 09, 2015 at 01:59:10PM +0100, Mason wrote: >> >>> Yesterday, I used /dev/mem to mmap 2 GB and (to my surprise) it worked. >>> Specifically, I opened /dev/mem O_RDWR | O_SYNC >>> then called >>> mmap(NULL, 1U<<31, PROT_WRITE, MAP_SHARED, fd, 0x80000000); >> >> So you asked to map 2GB starting at 2GB physical. >> >>> And mmap returned a valid pointer. >> >> And that mapping would have been created to map physical addresses >> 0x80000000-0xffffffff inclusive. >> >>> I was expecting it to fail. >>> >>> - the kernel is configured with VMSPLIT_3G (3G/1G user/kernel) >> >> This has no bearing on the above. > > I don't understand why. > > mmap allocates virtual addresses in the user-space process, yes? > So if I had VMSPLIT_2G, user-space processes would be limited > to 2G virtual addresses, and could not create a single 2G map > on top of its stack and text space. Or am I missing something? >
Because you are mmaping special file (dev/mem) mmap call is routed to the dedicated hook, responsible for all "magic" you see. Please, take a look at drivers/char/mem.c for details. Vladimir >>> - the kernel manages 256 MB RAM >>> - there is roughly 750 MB of VMALLOC space, no highmem >> >> vmalloc has no bearing on the above, mmap() doesn't allocate anything >> into vmalloc space. > > This means remap_pfn_range doesn't "put" anything in the kernel's > virtual address space. > >>> If I requested the same mapping *within the kernel* using ioremap, >>> would that fail because of limited VMALLOC space? >> >> Correct. > > OK. > >>> Moving to arch-specific questions (namely ARM Cortex-A9). >>> If I understand correctly (which is very possibly NOT the case) >>> the CPU has two registers pointing to page tables, one for >>> the current process, one for the kernel. And the CPU automatically >>> picks the correct one, based on the active context? >>> It would seem possible to have a full 4G for process, and a full 4G >>> for the kernel, using that method, no? (Like Ingo's old 4G/4G split). >>> Without the performance overhead of fiddling with the page tables. >>> What am I missing? >> >> It's possible to use both, but the CPU selects the page table register >> according to the virtual address. So it's not possible to have 4G for >> both. There's only a restricted set of options: 2G / 2G, where the >> bottom 2G of virtual space uses TTBR0 and the upper 2G uses TTBR1. >> 1G / 3G (1G for TTBR0, 3G for TTBR1). >> >> We don't use it because most people run with 3G for userspace, which >> isn't supported in hardware. > > I see. Thanks for spelling it out. > > Regards. > > _______________________________________________ > linux-arm-kernel mailing list > [email protected] > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel > > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
