linux-kernel - Re: [PATCH v2 3/4] arm64: mm: Don't remap pgtables for allocate vs populate

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Date: Thu, 11 Apr 2024 16:37:55 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Mark Rutland <mark.rutland@....com>
Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Ard Biesheuvel <ardb@...nel.org>, David Hildenbrand <david@...hat.com>,
 Donald Dutile <ddutile@...hat.com>, Eric Chanudet <echanude@...hat.com>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Itaru Kitayama <itaru.kitayama@...itsu.com>
Subject: Re: [PATCH v2 3/4] arm64: mm: Don't remap pgtables for allocate vs
 populate

On 11/04/2024 16:25, Mark Rutland wrote:
> On Thu, Apr 11, 2024 at 03:57:04PM +0100, Ryan Roberts wrote:
>> On 11/04/2024 15:48, Mark Rutland wrote:
>>> On Thu, Apr 11, 2024 at 02:37:49PM +0100, Ryan Roberts wrote:
>>>> On 11/04/2024 14:02, Mark Rutland wrote:
>>>>> but the logic remains fairly simple, and I suspect the overhead for late
>>>>> allocations might not matter since the bulk of late changes are non-allocating.
>>>>
>>>> Its just the fixmap overhead that remains...
>>>
>>> True; my thinking there is that almost all of the later changes are for smaller
>>> ranges than the linear map (~10s of MB vs GBs in your test data), so I'd expect
>>> the overhead of those to be dominated by the cost of mappin the linear map.
>>>
>>> The only big exception is arch_add_memory(), but memory hotplug is incredibly
>>> rare, and we're not making it massively slower than it already was...
>>
>> What about something like coco guest mem (or whatever its called). Isn't that
>> scrubbed out of the linear map? So if a coco VM is started with GBs of memory,
>> could that be a real case we want to optimize?
> 
> I think that's already handled -- the functions we have to carve portions out
> of the linear map use apply_to_page_range(), which doesn't use the fixmap. See
> set_memory_*() and set_direct_map_*() in arch/arm64/mm/pageattr.c.

Ahh gottya. Yet another table walker :)

> 
> Note that apply_to_page_range() does what its name implies and *only* handles
> mappings at page granularity. Hence not using that for
> mark_linear_text_alias_ro() and mark_rodata_ro() which need to be able to
> handle blocks.
> 
> Mark.