Re: [PATCH v4] arm64: mm: fix linear mem mapping access performance degradation

["guanghui.fgh" <guanghuifeng@linux.alibaba.com>]

Thanks.

在 2022/7/6 21:54, Mike Rapoport 写道:
> On Wed, Jul 06, 2022 at 11:04:24AM +0100, Catalin Marinas wrote:
>> On Tue, Jul 05, 2022 at 11:45:40PM +0300, Mike Rapoport wrote:
>>> On Tue, Jul 05, 2022 at 06:05:01PM +0100, Catalin Marinas wrote:
>>>> On Tue, Jul 05, 2022 at 06:57:53PM +0300, Mike Rapoport wrote:
>>>>> On Tue, Jul 05, 2022 at 04:34:09PM +0100, Catalin Marinas wrote:
>>>>>> On Tue, Jul 05, 2022 at 06:02:02PM +0300, Mike Rapoport wrote:
>>>>>>> +void __init remap_crashkernel(void)
>>>>>>> +{
>>>>>>> +#ifdef CONFIG_KEXEC_CORE
>>>>>>> +	phys_addr_t start, end, size;
>>>>>>> +	phys_addr_t aligned_start, aligned_end;
>>>>>>> +
>>>>>>> +	if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
>>>>>>> +	    return;
>>>>>>> +
>>>>>>> +	if (!crashk_res.end)
>>>>>>> +	    return;
>>>>>>> +
>>>>>>> +	start = crashk_res.start & PAGE_MASK;
>>>>>>> +	end = PAGE_ALIGN(crashk_res.end);
>>>>>>> +
>>>>>>> +	aligned_start = ALIGN_DOWN(crashk_res.start, PUD_SIZE);
>>>>>>> +	aligned_end = ALIGN(end, PUD_SIZE);
>>>>>>> +
>>>>>>> +	/* Clear PUDs containing crash kernel memory */
>>>>>>> +	unmap_hotplug_range(__phys_to_virt(aligned_start),
>>>>>>> +			    __phys_to_virt(aligned_end), false, NULL);
>>>>>>
>>>>>> What I don't understand is what happens if there's valid kernel data
>>>>>> between aligned_start and crashk_res.start (or the other end of the
>>>>>> range).
>>>>>
>>>>> Data shouldn't go anywhere :)
>>>>>
>>>>> There is
>>>>>
>>>>> +	/* map area from PUD start to start of crash kernel with large pages */
>>>>> +	size = start - aligned_start;
>>>>> +	__create_pgd_mapping(swapper_pg_dir, aligned_start,
>>>>> +			     __phys_to_virt(aligned_start),
>>>>> +			     size, PAGE_KERNEL, early_pgtable_alloc, 0);
>>>>>
>>>>> and
>>>>>
>>>>> +	/* map area from end of crash kernel to PUD end with large pages */
>>>>> +	size = aligned_end - end;
>>>>> +	__create_pgd_mapping(swapper_pg_dir, end, __phys_to_virt(end),
>>>>> +			     size, PAGE_KERNEL, early_pgtable_alloc, 0);
>>>>>
>>>>> after the unmap, so after we tear down a part of a linear map we
>>>>> immediately recreate it, just with a different page size.
>>>>>
>>>>> This all happens before SMP, so there is no concurrency at that point.
>>>>
>>>> That brief period of unmap worries me. The kernel text, data and stack
>>>> are all in the vmalloc space but any other (memblock) allocation to this
>>>> point may be in the unmapped range before and after the crashkernel
>>>> reservation. The interrupts are off, so I think the only allocation and
>>>> potential access that may go in this range is the page table itself. But
>>>> it looks fragile to me.
>>>
>>> I agree there are chances there will be an allocation from the unmapped
>>> range.
>>>
>>> We can make sure this won't happen, though. We can cap the memblock
>>> allocations with memblock_set_current_limit(aligned_end) or
>>> memblock_reserve(algined_start, aligned_end) until the mappings are
>>> restored.
>>
>> We can reserve the region just before unmapping to avoid new allocations
>> for the page tables but we can't do much about pages already allocated
>> prior to calling remap_crashkernel().
> 
> Right, this was bothering me too after I re-read you previous email.
> 
> One thing I can think of is to only remap the crash kernel memory if it is
> a part of an allocation that exactly fits into one ore more PUDs.
> 
> Say, in reserve_crashkernel() we try the memblock_phys_alloc() with
> PUD_SIZE as alignment and size rounded up to PUD_SIZE. If this allocation
> succeeds, we remap the entire area that now contains only memory allocated
> in reserve_crashkernel() and free the extra memory after remapping is done.
> If the large allocation fails, we fall back to the original size and
> alignment and don't allow unmapping crash kernel memory in
> arch_kexec_protect_crashkres().
>   
>> -- 
>> Catalin
> 
Thanks.

There is a new method.
I think we should use the patch v3(similar but need add some changes)

1.We can walk crashkernle block/section pagetable,
[[[(keep the origin block/section mapping valid]]]
rebuild the pte level page mapping for the crashkernel mem
rebuild left & right margin mem(which is in same block/section mapping 
but out of crashkernel mem) with block/section mapping

2.'replace' the origin block/section mapping by new builded mapping 
iterately

With this method, all the mem mapping keep valid all the time.

3.the patch v3 link:
https://lore.kernel.org/linux-mm/6dc308db-3685-4df5-506a-71f9e3794ec8@linux.alibaba.com/T/
(Need some changes)