[PATCH] um: implement arch_sync_kernel_mappings

[Anton Ivanov]

On 16/03/2021 10:34, Johannes Berg wrote:
> On Tue, 2021-03-16 at 10:30 +0000, Anton Ivanov wrote:
>
> On 16/03/2021 10:19, Johannes Berg wrote:
>> On Tue, 2021-03-16 at 10:02 +0000, Anton Ivanov wrote:
>>>> Want me to test it (without the kvmalloc)?
>>> Yes :)
>> This patch doesn't seem to improve vmalloc performance at all.
> Bugger...
>
> I was looking at it from this perspective.
>
> flush_cache_vmap is invoked out of here right after a
> map_kernel_range_noflush():
>
> https://elixir.bootlin.com/linux/latest/source/mm/vmalloc.c#L320
>
>
> If I understand the invocation correctly your patch results in a full
> flush for the mapped range each time it is mapped.
>
> Yes, that's how I understood/planned it.
>
>
> The invocation of map_kernel_range_noflush() actually has a built-in
> facility for a partial flush - it is exactly arch_sync_page_mappings and
> it is invoked here:
> https://elixir.bootlin.com/linux/latest/source/mm/vmalloc.c#L314
>
>
> Right, I saw this when you posted your patch and I looked how it would
> work.
>
> This makes the flush dependent on the level of modification - to be
> functionally equivalent to your patch it is an OR of all page levels
> which looked way too brutal :(
>
> That's why I tried it using only PMD. That looks like not enough :(
>
>
> Can you try tweaking the mask? Available masks are defined here:
> https://elixir.bootlin.com/linux/latest/source/include/linux/pgtable.h#L1474
>
> Ahhh.
>
>
> Well, I think you'd have to go for all of them. My workload is
> vmalloc'ing single pages only (for really tiny objects like 24 bytes, so
> it's stupid), so really you'd only get a single PTE changed, right?
>
> And then once you do that, it becomes equivalent to my patch (though
> actually done _more_ often since there might be cases where _noflush()
> is called and not flush_cache_vmap().

Ugh... OK. Let's stick with your approach.

I will +1 it in a minute.

Brgds,

>
> johannes
>
>
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-- 
Anton R. Ivanov
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