[PATCH] um: implement arch_sync_kernel_mappings

[Anton Ivanov]

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.

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

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

> 
> johannes
> 
> 
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Anton R. Ivanov
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