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

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Message-ID: <CA+CK2bBnbEtw7jL2fbukJ3aBCjn=-OVT70oEAsZ435vtFe18Vw@mail.gmail.com>
Date: Wed, 2 Apr 2025 14:37:12 -0400
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Pratyush Yadav <ptyadav@...zon.de>
Cc: Changyuan Lyu <changyuanl@...gle.com>, akpm@...ux-foundation.org, 
	anthony.yznaga@...cle.com, arnd@...db.de, ashish.kalra@....com, 
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	dwmw2@...radead.org, ebiederm@...ssion.com, graf@...zon.com, hpa@...or.com, 
	jgg@...dia.com, jgowans@...zon.com, kexec@...ts.infradead.org, 
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Subject: Re: [PATCH v5 09/16] kexec: enable KHO support for memory preservation

On Wed, Apr 2, 2025 at 12:47 PM Pratyush Yadav <ptyadav@...zon.de> wrote:
>
> 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!

Hi Pratyush,

Just to clarify, how soon? We are about to post v6 for KHO, with all
other comments in this thread addressed.

Thanks,
Pasha

>
> --
> Regards,
> Pratyush Yadav