linux-kernel - Re: [PATCH v5 09/16] kexec: enable KHO support for memory preservation

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Message-ID: <mafs0ecyaqzmd.fsf_-_@amazon.de>
Date: Wed, 2 Apr 2025 16:47:38 +0000
From: Pratyush Yadav <ptyadav@...zon.de>
To: Changyuan Lyu <changyuanl@...gle.com>
CC: <akpm@...ux-foundation.org>, <anthony.yznaga@...cle.com>, <arnd@...db.de>,
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Subject: Re: [PATCH v5 09/16] kexec: enable KHO support for memory preservation

Hi,

On Wed, Apr 02 2025, Changyuan Lyu wrote:

> Hi Pratyush, Thanks for suggestions!
>
> On Thu, Mar 27, 2025 at 17:28:40 +0000, Pratyush Yadav <ptyadav@...zon.de> wrote:
>> On Thu, Mar 27 2025, Jason Gunthorpe wrote:
>>
>> > On Thu, Mar 27, 2025 at 10:03:17AM +0000, Pratyush Yadav wrote:
>> >
>> >> Of course, with the current linked list structure, this cannot work. But
>> >> I don't see why we need to have it. I think having a page-table like
>> >> structure would be better -- only instead of having PTEs at the lowest
>> >> levels, you have the bitmap.
>> >
>> > Yes, but there is a trade off here of what I could write in 30 mins
>> > and what is maximally possible :) The xarray is providing a page table
>> > implementation in a library form.
>> >
>> > I think this whole thing can be optimized, especially the
>> > memblock_reserve side, but the idea here is to get started and once we
>> > have some data on what the actual preservation workload is then
>> > someone can optimize this.
>> >
>> > Otherwise we are going to be spending months just polishing this one
>> > patch without any actual data on where the performance issues and hot
>> > spots actually are.
>>
>> The memblock_reserve side we can optimize later, I agree. But the memory
>> preservation format is ABI and I think that is worth spending a little
>> more time on. And I don't think it should be that much more complex than
>> the current format.
>>
>> I want to hack around with it, so I'll give it a try over the next few
>> days and see what I can come up with.
>
> I agree with Jason that "nothing is ABI at this
> point" and it will take some time for KHO to stabilize.
>
> On the other hand if you have already came up with something working and
> simple, we can include it in the next version.

I already have something that works with zero-order pages. I am
currently implementing support for other orders. It is almost done, but
I need to test it and do a performance comparison with the current
patch. Will post something soon!

-- 
Regards,
Pratyush Yadav