linux-kernel - Re: [PATCH v3 29/30] luo: allow preserving memfd

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Message-ID: <CAF8kJuMcD4HA_CjK7iZ_47jgnu63pF-0WRrhdREvUmMVOgWBEg@mail.gmail.com>
Date: Thu, 28 Aug 2025 16:00:18 -0700
From: Chris Li <chrisl@...nel.org>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Pratyush Yadav <pratyush@...nel.org>, Pasha Tatashin <pasha.tatashin@...een.com>, jasonmiu@...gle.com, 
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Subject: Re: [PATCH v3 29/30] luo: allow preserving memfd

On Thu, Aug 28, 2025 at 5:43 AM Jason Gunthorpe <jgg@...dia.com> wrote:
>
> On Wed, Aug 27, 2025 at 05:03:55PM +0200, Pratyush Yadav wrote:
>
> > I think we need something a luo_xarray data structure that users like
> > memfd (and later hugetlb and guest_memfd and maybe others) can build to
> > make serialization easier. It will cover both contiguous arrays and
> > arrays with some holes in them.
>
> I'm not sure xarray is the right way to go, it is very complex data
> structure and building a kho variation of it seems like it is a huge
> amount of work.
>
> I'd stick with simple kvalloc type approaches until we really run into
> trouble.
>
> You can always map a sparse xarray into a kvalloc linear list by
> including the xarray index in each entry.

Each entry will be 16 byte, 8 for index and 8 for XAvalue, right?

> Especially for memfd where we don't actually expect any sparsity in
> real uses cases there is no reason to invest a huge effort to optimize
> for it..

Ack.

>
> > As I explained above, the versioning is already there. Beyond that, why
> > do you think a raw C struct is better than FDT? It is just another way
> > of expressing the same information. FDT is a bit more cumbersome to
> > write and read, but comes at the benefit of more introspect-ability.
>
> Doesn't have the size limitations, is easier to work list, runs
> faster.

Yes, especially when you have a large array.

Chris