vmalloc_reserve with no highmem

vmalloc_reserve with no highmem

[Florian Fainelli]

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

in sanity_check_meminfo(), high_memory is unconditionally assigned with
arm_lowmem_limit's VA.

The following quick and dirty patch seems to do it for me, but I am not
confident this is remotely the correct approach here:

diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
index 14428d2..e196ea4 100644
--- a/arch/arm/mm/mmu.c
+++ b/arch/arm/mm/mmu.c
@@ -1196,7 +1211,14 @@ void __init sanity_check_meminfo(void)
        }
 #endif
        meminfo.nr_banks = j;
-       high_memory = __va(arm_lowmem_limit - 1) + 1;
+
+       if (vmalloc_limit > arm_lowmem_limit)
+               high_memory = vmalloc_min - VMALLOC_OFFSET;
+       else
+               high_memory = __va(arm_lowmem_limit - 1) + 1;
+
+       pr_info("%s: high_memory: 0x%p, arm_lowmem_limit: 0x%llx\n",
+               __func__, high_memory, arm_lowmem_limit);

BTW, even when there is highmem available, setting vmalloc= on the
command-line is off by VMALLOC_OFFSET, the way early_vmalloc() computes
things, is that intended?

Thanks!
-- 
Florian

vmalloc_reserve with no highmem

[Laura Abbott]

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
>
> in sanity_check_meminfo(), high_memory is unconditionally assigned with
> arm_lowmem_limit's VA.
>
> The following quick and dirty patch seems to do it for me, but I am not
> confident this is remotely the correct approach here:
>
> diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
> index 14428d2..e196ea4 100644
> --- a/arch/arm/mm/mmu.c
> +++ b/arch/arm/mm/mmu.c
> @@ -1196,7 +1211,14 @@ void __init sanity_check_meminfo(void)
>          }
>   #endif
>          meminfo.nr_banks = j;
> -       high_memory = __va(arm_lowmem_limit - 1) + 1;
> +
> +       if (vmalloc_limit > arm_lowmem_limit)
> +               high_memory = vmalloc_min - VMALLOC_OFFSET;
> +       else
> +               high_memory = __va(arm_lowmem_limit - 1) + 1;
> +
> +       pr_info("%s: high_memory: 0x%p, arm_lowmem_limit: 0x%llx\n",
> +               __func__, high_memory, arm_lowmem_limit);
>
> BTW, even when there is highmem available, setting vmalloc= on the
> command-line is off by VMALLOC_OFFSET, the way early_vmalloc() computes
> things, is that intended?
>
> Thanks!
>

Which baseline are you testing off of? Meminfo was removed
a while ago so I'm not sure where the meminfo.nr_banks context is
coming from.

This is how arm currently defines VMALLOC ranges

#define VMALLOC_OFFSET          (8*1024*1024)
#define VMALLOC_START           (((unsigned long)high_memory + VMALLOC_OFFSET) & ~(VMALLOC_OFFSET-1))
#define VMALLOC_END             0xff000000UL

VMALLOC_START is based off of the value of high_memory. high_memory
is supposed to be the limit of the kernel direct mapped ram region so setting
it to something above that is asking for trouble. It seems like this is
working as intended. You'd have to decouple VMALLOC_START and
high_memory to make this work properly.

Having a vmalloc_start be greater than the end of direct mapped RAM seems a
little unusual. This would be leaving a hole in the virtual space. Is this
what you are intending or is there another use case for adjusting the
vmalloc space as you described?

Thanks,
Laura

vmalloc_reserve with no highmem

[Florian Fainelli]

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
>>
>> in sanity_check_meminfo(), high_memory is unconditionally assigned with
>> arm_lowmem_limit's VA.
>>
>> The following quick and dirty patch seems to do it for me, but I am not
>> confident this is remotely the correct approach here:
>>
>> diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
>> index 14428d2..e196ea4 100644
>> --- a/arch/arm/mm/mmu.c
>> +++ b/arch/arm/mm/mmu.c
>> @@ -1196,7 +1211,14 @@ void __init sanity_check_meminfo(void)
>>          }
>>   #endif
>>          meminfo.nr_banks = j;
>> -       high_memory = __va(arm_lowmem_limit - 1) + 1;
>> +
>> +       if (vmalloc_limit > arm_lowmem_limit)
>> +               high_memory = vmalloc_min - VMALLOC_OFFSET;
>> +       else
>> +               high_memory = __va(arm_lowmem_limit - 1) + 1;
>> +
>> +       pr_info("%s: high_memory: 0x%p, arm_lowmem_limit: 0x%llx\n",
>> +               __func__, high_memory, arm_lowmem_limit);
>>
>> BTW, even when there is highmem available, setting vmalloc= on the
>> command-line is off by VMALLOC_OFFSET, the way early_vmalloc() computes
>> things, is that intended?
>>
>> Thanks!
>>
> 
> Which baseline are you testing off of? Meminfo was removed
> a while ago so I'm not sure where the meminfo.nr_banks context is
> coming from.

This is 3.14, but I can forward port and test the change on 4.3-rcX as
well where this is still relevant.

> 
> This is how arm currently defines VMALLOC ranges
> 
> #define VMALLOC_OFFSET          (8*1024*1024)
> #define VMALLOC_START           (((unsigned long)high_memory +
> VMALLOC_OFFSET) & ~(VMALLOC_OFFSET-1))
> #define VMALLOC_END             0xff000000UL
> 
> VMALLOC_START is based off of the value of high_memory. high_memory
> is supposed to be the limit of the kernel direct mapped ram region so
> setting it to something above that is asking for trouble.

You're right, that does not seem right.

> It seems like this is
> working as intended. You'd have to decouple VMALLOC_START and
> high_memory to make this work properly.

That does not sound that trivial, but I can probably tinker with
something locally which is not keyed of high_memory for now.

> 
> Having a vmalloc_start be greater than the end of direct mapped RAM seems a
> little unusual. This would be leaving a hole in the virtual space. Is this
> what you are intending or is there another use case for adjusting the
> vmalloc space as you described?

The intended use case is something like you have 512MB of physical
memory, you want to give 248MB of this for vmalloc and no more, and the
remainder to a statically carved out region, used for non Linux-managed
allocations.

Do we agree though, that if you are setting vmalloc on the command-line
we should be reserving the amount that is being indicated?
-- 
Florian

vmalloc_reserve with no highmem

[Russell King - ARM Linux]

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.

> > #define VMALLOC_OFFSET          (8*1024*1024)
> > #define VMALLOC_START           (((unsigned long)high_memory +
> > VMALLOC_OFFSET) & ~(VMALLOC_OFFSET-1))
> > #define VMALLOC_END             0xff000000UL
> > 
> > VMALLOC_START is based off of the value of high_memory. high_memory
> > is supposed to be the limit of the kernel direct mapped ram region so
> > setting it to something above that is asking for trouble.
> 
> You're right, that does not seem right.

That's correct.  high_memory is the last address of mapped lowmem.
We then have an 8MB space.  The quote above is correct.

I don't see what the problem is.

-- 
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.

vmalloc_reserve with no highmem

[Laura Abbott]

On 10/20/2015 12:17 PM, 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
>>>
>>> in sanity_check_meminfo(), high_memory is unconditionally assigned with
>>> arm_lowmem_limit's VA.
>>>
>>> The following quick and dirty patch seems to do it for me, but I am not
>>> confident this is remotely the correct approach here:
>>>
>>> diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
>>> index 14428d2..e196ea4 100644
>>> --- a/arch/arm/mm/mmu.c
>>> +++ b/arch/arm/mm/mmu.c
>>> @@ -1196,7 +1211,14 @@ void __init sanity_check_meminfo(void)
>>>           }
>>>    #endif
>>>           meminfo.nr_banks = j;
>>> -       high_memory = __va(arm_lowmem_limit - 1) + 1;
>>> +
>>> +       if (vmalloc_limit > arm_lowmem_limit)
>>> +               high_memory = vmalloc_min - VMALLOC_OFFSET;
>>> +       else
>>> +               high_memory = __va(arm_lowmem_limit - 1) + 1;
>>> +
>>> +       pr_info("%s: high_memory: 0x%p, arm_lowmem_limit: 0x%llx\n",
>>> +               __func__, high_memory, arm_lowmem_limit);
>>>
>>> BTW, even when there is highmem available, setting vmalloc= on the
>>> command-line is off by VMALLOC_OFFSET, the way early_vmalloc() computes
>>> things, is that intended?
>>>
>>> Thanks!
>>>
>>
>> Which baseline are you testing off of? Meminfo was removed
>> a while ago so I'm not sure where the meminfo.nr_banks context is
>> coming from.
>
> This is 3.14, but I can forward port and test the change on 4.3-rcX as
> well where this is still relevant.
>
>>
>> This is how arm currently defines VMALLOC ranges
>>
>> #define VMALLOC_OFFSET          (8*1024*1024)
>> #define VMALLOC_START           (((unsigned long)high_memory +
>> VMALLOC_OFFSET) & ~(VMALLOC_OFFSET-1))
>> #define VMALLOC_END             0xff000000UL
>>
>> VMALLOC_START is based off of the value of high_memory. high_memory
>> is supposed to be the limit of the kernel direct mapped ram region so
>> setting it to something above that is asking for trouble.
>
> You're right, that does not seem right.
>
>> It seems like this is
>> working as intended. You'd have to decouple VMALLOC_START and
>> high_memory to make this work properly.
>
> That does not sound that trivial, but I can probably tinker with
> something locally which is not keyed of high_memory for now.
>
>>
>> Having a vmalloc_start be greater than the end of direct mapped RAM seems a
>> little unusual. This would be leaving a hole in the virtual space. Is this
>> what you are intending or is there another use case for adjusting the
>> vmalloc space as you described?
>
> The intended use case is something like you have 512MB of physical
> memory, you want to give 248MB of this for vmalloc and no more, and the
> remainder to a statically carved out region, used for non Linux-managed
> allocations.

I'm confused here. What does 512MB of physical memory have to do with your
virtual address space? I think what you are saying is that you have 512MB
of physical memory and you want to carve out 248M of that physical memory
for a non-Linux region. If that's the case you would be better off reserving
the physical region in devicetree. The virtual space wouldn't be relevant
for anything outside of Linux.

Thanks,
Laura

vmalloc_reserve with no highmem

[Florian Fainelli]

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.

> 
>>> #define VMALLOC_OFFSET          (8*1024*1024)
>>> #define VMALLOC_START           (((unsigned long)high_memory +
>>> VMALLOC_OFFSET) & ~(VMALLOC_OFFSET-1))
>>> #define VMALLOC_END             0xff000000UL
>>>
>>> VMALLOC_START is based off of the value of high_memory. high_memory
>>> is supposed to be the limit of the kernel direct mapped ram region so
>>> setting it to something above that is asking for trouble.
>>
>> You're right, that does not seem right.
> 
> That's correct.  high_memory is the last address of mapped lowmem.
> We then have an 8MB space.  The quote above is correct.
> 
> I don't see what the problem is.

Might just be documentation vs. actual results displayed by the "Kernel
virtual memory layout". A similar "issue" exists if you specify vmalloc=
on the command-line and you get a result which is offset by VMALLOC_OFFSET.

I will do a better homework and go back to thinking about this.
-- 
Florian

vmalloc_reserve with no highmem

[Russell King - ARM Linux]

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.

> Might just be documentation vs. actual results displayed by the "Kernel
> virtual memory layout". A similar "issue" exists if you specify vmalloc=
> on the command-line and you get a result which is offset by VMALLOC_OFFSET.
> 
> I will do a better homework and go back to thinking about this.

x86 also has an 8MB hole.

-- 
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.

vmalloc_reserve with no highmem

[Gregory Fong]

On Tue, Oct 20, 2015 at 12:40 PM, Russell King - ARM Linux
<linux@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.