Re: [PATCH] [RFC] arm64: mmu: use range based TLB flushing when hot unplugging memory

[Ryan Roberts <ryan.roberts@arm.com>]

On 21/05/2026 11:46, Balbir Singh wrote:
> On Thu, May 21, 2026 at 09:50:04AM +0100, Ryan Roberts wrote:
>> On 21/05/2026 05:24, Alistair Popple wrote:
>>> Hot unplugging memory on ARM64 requires a TLB invalidate after unmapping
>>> the page to be hot unplugged from the direct map. Currently that happens
>>> one page at a time, meaning range based invalidates cannot be used. The
>>> result of this is that removing large amounts of memory takes a long
>>> time and in some cases can trigger an RCU stall warning.
>>>
>>> For example on one system hot unplugging 480GB of memory takes ~1
>>> minute. With this change the same operation took ~1 second, a 60x
>>> improvement.
>>>
>>> Signed-off-by: Alistair Popple <apopple@nvidia.com>
>>>
>>> ---
>>>
>>> This is an RFC, because I'm not sure the change is correct as it frees
>>> the PTE page before flushing the TLB. I'm not familiar enough with ARM64
>>> architecture to be sure this is safe, for example I don't know if HW
>>> can update PTE bits such as access/dirty in the page through a stale
>>> TLB entry.
>>>
>>> If so this would open a window during which the page is free but could
>>> still be written to. Likely the safe option would be to collect all the
>>> pages to be free on a list and free them after doing the range based TLB
>>> flush, but wanted to get feedback on the approach before implementing it
>>> which is the goal of this RFC.
>>
>> Hi Alistair,
>>
>> This patch doesn't apply on v7.1-rc4 because it conflicts with this patch:
>>
>> Commit 48478b9f79137 ("arm64/mm: Enable batched TLB flush in unmap_hotplug_range()")
>>
>> which has a very similar performance improvement, so hopefully it solves your
>> problem?
>>
>> There are two paths which use this logic; unmapping the linear map and unmapping
>> the corresponding vmemmap. In the latter case, the memory is also freed, so we
>> can't safely do the range optimizaiton there since the TLB needs to be flushed
>> before freeing the memory. But the linear map is the big, slow bit so hopefully
>> it's sufficent for you?
>>
> 
> I assume vmemmap path is for tearing down the struct pages corresponding
> to the physical memory and vmemmap teardowns taking a flush should be
> OK. It is worth checking if the issue is already fixed.

Yes, exactly. Assuming you're using 64K pages, I think vmemmap is mapped using
base pages. So you'd get 1024 struct pages on a page, so 1 TLBI per 64M of
memory that is being unplugged on the vmemmap path.

On the linear map path, it's a single range tlbi operation to over the entire range.

Alistair's patch is doing it per 512M in both cases, but is unsafe for vmemmap
as currently written.

> 
> Balbir
> <snip>