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SubjectRe: [PATCH part2 v2 0/8] Arrange hotpluggable memory as ZONE_MOVABLE
Hello tejun,

On 10/14/2013 11:19 PM, Tejun Heo wrote:
> Hey,
>
> On Mon, Oct 14, 2013 at 11:06:14PM +0800, Zhang Yanfei wrote:
>> a little difference here, consider a 16-GB node. If we parse SRAT earlier,
>> and still use the top-down allocation, and kernel image is loaded at 16MB,
>> we reserve other nodes but this 16GB node that kernel resides in is used
>> for boot-up allocation. So the page table is allocated from 16GB to 0.
>> The page table is allocated on top of the the memory as possible.
>>
>> But if we use this approach, no matter how large the page table is, we
>> allocate the page table in low memory which is the case that hpa concerns
>> about the DMA.
>
> Yeah, sure there will be cases where parsing SRAT would be better.
>
> 4k mapping is in use, which is mostly for debugging && memory map is
> composed such that the highest non-hotpluggable address is high
> enough.
>
> It's going in circles again but my point has always been that the
> above in itself don't seem to be substantial enough to justify
> putting, say, initrd loading before page table init.
>
> Later some argued that bringing SRAT parsing earlier could help
> implementing finer grained hotplug, which would be an acceptable path
> to follow; however, that doesn't turn out to be true either.
>
> * Again, it matter if and only if 4k mapping is in use. Do we even
> care?
>
> * SRAT isn't enough. The whole device tree needs to be parsed to put
> page tables into local device. It's a lot of churn, including major
> updates to page table allocation, just to support debug 4k mapping
> cases. Doesn't make much sense to me.
>
> So, SRAT's usefulness seems extremely limited - it helps if the user
> wants to use debug features along with memory hotplug on an extreme
> large machine with devices which have low DMA limit, and that's it.
> To me, it seems to be a poor argument. Just declaring memory hotplug
> works iff large kernel mapping is in use feels like a pretty good
> trade-off to me, and I have no idea why I have to repeat all this,
> which I've written multiple times already, in a private thread again.
>
> If the thread is to make progress, one has to provide counter
> arguments to the points raised. It feels like I'm going in circle
> again. The exact same content I wrote above has been repeated
> multiple times in the past discussions and I'm getting tired of doing
> it without getting any actual response.
>
> When replying, please restore cc's and keep the whole body.
>

Thanks for the whole explanation again. I was just raising some argument
that other guys raised before. I agree with what you said above and already
put some of them into the patch 4 description in v7 version.

Now could you please help reviewing the part2? As I said before, no matter
how we implement the part1, part2 is kind of independent.

--
Thanks.
Zhang Yanfei


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