linux-kernel - Re: [PATCH 1/3] memory: extern memory_block_size_bytes and set_memory_block_size

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Message-ID: <01fbdcef-b923-4bb0-80b0-f1d3e57fe515@redhat.com>
Date: Mon, 14 Oct 2024 22:32:36 +0200
From: David Hildenbrand <david@...hat.com>
To: Gregory Price <gourry@...rry.net>
Cc: linux-cxl@...r.kernel.org, x86@...nel.org, linux-mm@...ck.org,
 linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
 dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
 osalvador@...e.de, gregkh@...uxfoundation.org, rafael@...nel.org,
 akpm@...ux-foundation.org, dan.j.williams@...el.com,
 Jonathan.Cameron@...wei.com, alison.schofield@...el.com, rrichter@....com,
 terry.bowman@....com, lenb@...nel.org, dave.jiang@...el.com,
 ira.weiny@...el.com
Subject: Re: [PATCH 1/3] memory: extern memory_block_size_bytes and
 set_memory_block_size_order

On 14.10.24 16:25, Gregory Price wrote:
> On Mon, Oct 14, 2024 at 01:54:27PM +0200, David Hildenbrand wrote:
>> On 08.10.24 17:21, Gregory Price wrote:
>>> On Tue, Oct 08, 2024 at 05:02:33PM +0200, David Hildenbrand wrote:
>>>> On 08.10.24 16:51, Gregory Price wrote:
>>>>>>> +int __weak set_memory_block_size_order(unsigned int order)
>>>>>>> +{
>>>>>>> +	return -ENODEV;
>>>>>>> +}
>>>>>>> +EXPORT_SYMBOL_GPL(set_memory_block_size_order);
>>>>>>
>>>>>> I can understand what you are trying to achieve, but letting arbitrary
>>>>>> modules mess with this sounds like a bad idea.
>>>>>>
>>>>>
>>>>> I suppose the alternative is trying to scan the CEDT from inside each
>>>>> machine, rather than the ACPI driver?  Seems less maintainable.
>>>>>
>>>>> I don't entirely disagree with your comment.  I hummed and hawwed over
>>>>> externing this - hence the warning in the x86 machine.
>>>>>
>>>>> Open to better answers.
>>>>
>>>> Maybe an interface to add more restrictions on the maximum size might be
>>>> better (instead of setting the size/order, you would impose another upper
>>>> limit).
>>>
>>> That is effectively what set_memory_block_size_order is, though.  Once
>>> blocks are exposed to the allocators, its no longer safe to change the
>>> size (in part because it was built assuming it wouldn't change, but I
>>> imagine there are other dragons waiting in the shadows to bite me).
>>
>> Yes, we must run very early.
>>
>> How is this supposed to interact with code like
>>
>> set_block_size()
>>
>> that also calls set_memory_block_size_order() on UV systems (assuming there
>> will be CXL support sooner or later?)?
>>
>>
> 
> Tying the other email to this one - just clarifying the way forward here.
> 
> It sounds like you're saying at a minimum drop EXPORT tags to prevent
> modules from calling it - but it also sounds like built-ins need to be
> prevented from touching it as well after a certain point in early boot.

Right, at least the EXPORT is not required.

> 
> Do you think I should go down the advise() path as suggested by Ira,
> just adding a arch_lock_blocksize() bit and have set_..._order check it,
> or should we just move towards each architecture having to go through
> the ACPI:CEDT itself?

Let's summarize what we currently have on x86 is:

1) probe_memory_block_size()

Triggered on first memory_block_size_bytes() invocation. Makes a 
decision based on:

a) Already set size using set_memory_block_size_order()
b) RAM size
c) Bare metal vs. virt (bare metal -> use max)
d) Virt: largest block size aligned to memory end


2) set_memory_block_size_order()

Triggered by set_block_size() on UV systems.


I don't think set_memory_block_size_order() is the right tool to use. We 
just want to leave that alone I think -- it's a direct translation of a 
kernel cmdline parameter that should win.

You essentially want to tweak the b)->d) logic to take other alignment 
into consideration.

Maybe have some simple callback mechanism probe_memory_block_size() that 
can consult other sources for alignment requirements?

If that's not an option, then another way to set further min-alignment 
requirements (whereby we take MIN(old_align, new_align))?

-- 
Cheers,

David / dhildenb