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

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Message-ID: <20250902113857.GB186519@nvidia.com>
Date: Tue, 2 Sep 2025 08:38:57 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: Pratyush Yadav <pratyush@...nel.org>, Mike Rapoport <rppt@...nel.org>,
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Subject: Re: [PATCH v3 29/30] luo: allow preserving memfd

On Mon, Sep 01, 2025 at 07:02:46PM +0000, Pasha Tatashin wrote:
> > >> > This really wants some luo helper
> > >> >
> > >> > 'luo alloc array'
> > >> > 'luo restore array'
> > >> > 'luo free array'
> > >>
> > >> We can just add kho_{preserve,restore}_vmalloc(). I've drafted it here:
> > >> https://git.kernel.org/pub/scm/linux/kernel/git/rppt/linux.git/log/?h=kho/vmalloc/v1
> > >
> > > The patch looks okay to me, but it doesn't support holes in vmap
> > > areas. While that is likely acceptable for vmalloc, it could be a
> > > problem if we want to preserve memfd with holes and using vmap
> > > preservation as a method, which would require a different approach.
> > > Still, this would help with preserving memfd.
> >
> > I agree. I think we should do it the other way round. Build a sparse
> > array first, and then use that to build vmap preservation. Our emails
> 
> Yes, sparse array support would help both: vmalloc and memfd preservation.

Why? vmalloc is always full popoulated, no sparseness..

And again in real systems we expect memfd to be fully populated too.

I wouldn't invest any time in something like this right now. Just be
inefficient if there is sparseness for some reason.

Jason