vmalloc_reserve with no highmem

[Gregory Fong]

On Tue, Oct 20, 2015 at 12:40 PM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Tue, Oct 20, 2015 at 12:32:04PM -0700, Florian Fainelli wrote:
>> On 20/10/15 12:25, Russell King - ARM Linux wrote:
>> > On Tue, Oct 20, 2015 at 12:17:59PM -0700, Florian Fainelli wrote:
>> >> On 20/10/15 11:52, Laura Abbott wrote:
>> >>> On 10/19/2015 04:20 PM, Florian Fainelli wrote:
>> >>>> Hi Russell, Laura,
>> >>>>
>> >>>> Setting vmalloc= on the kernel command-line to define the amount of
>> >>>> vmalloc_reserve is not quite working when you have no highmem, as is the
>> >>>> case of one my boards which has 512MB or 256M populated on a first bank
>> >>>> at PA 0x0.
>> >>>>
>> >>>> What happens in that case is that, despite setting vmalloc_reserve,
>> >>>> therefore bumping up vmalloc_min to a higher address than high_memory,
>> >>>> which is assigned __va(arm_lowmem_limit), we end-up with VMALLOC_START
>> >>>> at high_memory + VMALLOC_OFFSET, which yields the amount of physical
>> >>>> memory (start at PA 0x0 in my case) - VMALLOC_OFFSET.
>> >>>>
>> >>>> The maths look like this for this particular board (512MB)
>> >>>>
>> >>>> high_memory = 0x20000000 + PAGE_OFFSET = 0x20000000 + 0xC0000000 =
>> >>>> 0xE0000000
>> >>>> vmalloc_min = 0xFF00000 - (248 * 1024 * 1024) = 0xEF800000
>> >>>>
>> >>>> so we end-up with VMALLOC_START = high_memory + VMALLOC_OFFSET =
>> >>>> 0xE0000000 + 8 * 1024* 1024 = 0xE0800000
>> >
>> > Correct, high_memory will be 0xe0000000, which will set VMALLOC_START
>> > to 0xe0800000, and you'll have lots of vmalloc space available.  That's
>> > intentional.
>> >
>> > vmalloc= sets the _minimum_ vmalloc space that's available, not the
>> > absolute amount of space.
>>
>> I think we need to update Documentation/kernel-parameters.txt then:
>>
>> """
>>         vmalloc=nn[KMG] [KNL,BOOT] Forces the vmalloc area to have an exact
>>                         size of <nn>. This can be used to increase the
>>                         minimum size (128MB on x86). It can also be used to
>>                         decrease the size and leave more room for directly
>>                         mapped kernel RAM.
>> """
>>
>> this makes me understand I should get the absolute value I specified.
>
> It's not very well worded, but:
>
> "This can be used to increase the minimum size" is the applicable part
> here.  On ARM we follow x86.

It isn't clear from the documentation that there might be some offset
that needs to be taken into account.  What would you think of a change
in wording to:

vmalloc=nn[KMG] [KNL,BOOT] Forces the vmalloc area to have an exact
                        size of <nn>. This can be used to increase the
                        minimum size (128MB on x86). It can also be used to
                        decrease the size and leave more room for directly
                        mapped kernel RAM.  On some architectures, the
                        vmalloc area includes an offset, resulting in the
                        available vmalloc space being smaller than the size
                        specified here.