Messages in this thread | | | Date | Fri, 09 Jan 2015 19:42:58 +0100 | From | Mason <> | Subject | Re: ioremap vs remap_pfn_range, VMSPLIT, etc |
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Hello Vladimir,
On 09/01/2015 19:06, Vladimir Murzin wrote: > 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.
Errr, I thought it was clear I had read the source ;-) ("I know /dev/mem's mmap calls remap_pfn_range [...]") http://lxr.free-electrons.com/source/drivers/char/mem.c#L307
Hence my ioremap vs remap_pfn_range subject ;-)
So are you saying I could use remap_pfn_range to map the full 4G of PA space into a process's VA space?
Or, if I picked the VMSPLIT_2G option, are you saying mmap'ing 2G would succeed? (I will test these two scenarios ASAP.)
Regards.
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