Re: [PATCH v2] vmalloc: Modify the alloc_vmap_area() error message for better diagnostics

Re: [PATCH v2] vmalloc: Modify the alloc_vmap_area() error message for better diagnostics

[Christoph Lameter (Ampere)]

On Tue, 4 Jun 2024, Matthew Wilcox wrote:

> On Mon, Jun 03, 2024 at 09:30:54PM +0000, Shubhang Kaushik OS wrote:
>> 'vmap allocation for size %lu failed: use vmalloc=<size> to increase size'
>> The above warning is seen in the kernel functionality for allocation of
>> the restricted virtual memory range till exhaustion.
>>
>> This message is misleading because 'vmalloc=' is not a valid kernel
>> parameter on a number of platforms, in particular it is not supported
>> on arm64. With the update, the output gets modified to include the function
>
> Why not fix arm64?

Arm64 does not need vmalloc= tuning and the problem is not related to 
being out of vmalloc space in general.

This occurs if the virtual range during a vmalloc was restricted and is 
not available (f.e. if one wants a module to be loaded in optimal branch 
distance to the kernel text segment and we loaded too many modules). The 
error message needs to indicate the virtual memory restriction which helps 
the developer/user to debug the situation and not create a wild goose 
chase for a kernel parmaeter that does not exist.

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Re: [PATCH v2] vmalloc: Modify the alloc_vmap_area() error message for better diagnostics

[Anshuman Khandual]

On 6/4/24 05:46, Christoph Lameter (Ampere) wrote:
> On Tue, 4 Jun 2024, Matthew Wilcox wrote:
> 
>> On Mon, Jun 03, 2024 at 09:30:54PM +0000, Shubhang Kaushik OS wrote:
>>> 'vmap allocation for size %lu failed: use vmalloc=<size> to increase size'
>>> The above warning is seen in the kernel functionality for allocation of
>>> the restricted virtual memory range till exhaustion.
>>>
>>> This message is misleading because 'vmalloc=' is not a valid kernel
>>> parameter on a number of platforms, in particular it is not supported
>>> on arm64. With the update, the output gets modified to include the function
>>
>> Why not fix arm64?
> 
> Arm64 does not need vmalloc= tuning and the problem is not related to being out of vmalloc space in general.
> 
> This occurs if the virtual range during a vmalloc was restricted and is not available (f.e. if one wants a module to be loaded in optimal branch distance to the kernel text segment and we loaded too many modules). The error message needs to indicate the virtual memory restriction which helps the developer/user to debug the situation and not create a wild goose chase for a kernel parmaeter that does not exist.

Agreed, current warning message here is misleading pointing to a non-existent
kernel command line parameter on the given platform.

This kernel parameter i.e 'vmalloc=' seems to be supported only on the x86 and
arm (32) platforms. Should not Documentation/admin-guide/kernel-parameters.txt
be updated as well making this bit clear ?

> 
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