linux-kernel - Re: [PATCH 08/16] mm: add remap_pfn_range_prepare(), remap_pfn_range

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Message-ID: <34d93f7f-8bb8-4ffc-a6b9-05b68e876766@lucifer.local>
Date: Mon, 8 Sep 2025 15:18:46 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
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Subject: Re: [PATCH 08/16] mm: add remap_pfn_range_prepare(),
 remap_pfn_range_complete()

On Mon, Sep 08, 2025 at 10:35:38AM -0300, Jason Gunthorpe wrote:
> On Mon, Sep 08, 2025 at 02:27:12PM +0100, Lorenzo Stoakes wrote:
>
> > It's not only remap that is a concern here, people do all kinds of weird
> > and wonderful things in .mmap(), sometimes in combination with remap.
>
> So it should really not be split this way, complete is a badly name

I don't understand, you think we can avoid splitting this in two? If so, I
disagree.

We have two stages, _intentionally_ placed to avoid the issues the mmap_prepare
series in the first instance worked to avoid:

1. 'Hey, how do we configure this VMA we have _not yet set up_'
2. 'OK it's set up, now do you want to do something else?

I'm sorry but I'm not sure how we could otherwise do this.

Keep in mind re: point 1, we _need_ the VMA to be established enough to check
for merge etc.

Another key aim of this change was to eliminate the need for a merge re-check.

> prepopulate and it should only fill the PTEs, which shouldn't need
> more locking.
>
> The only example in this series didn't actually need to hold the lock.

There's ~250 more mmap callbacks to work through. Do you provide a guarantee
that:

- All 250 absolutely only need access to the VMAs to perform prepopulation of
  this nature.

- That absolutely none will set up state in the prepopulate step that might need
  to be unwound should an error arise?

Keeping in mind I must remain practical re: refactoring each caller.

I mean, let me go check what you say re: the resctl lock, if you're right I
could drop mmap_abort for now and add it later if needed.

But re: calling mmap_complete prepopulate, I don't really think that's sensible.

mmap_prepare is invoked at the point of the preparation of the mapping, and
mmap_complete is invoked once that preoparation is complete to allow further
actions.

I'm obviously open to naming suggestions, but I think it's safer to consistently
refer to where we are in the lifecycle rather than presuming what the caller
might do.

(I'd _prefer_ they always did just prepopulate, but I just don't think we
necessarily can).

>
> Jason

Cheers, Lorenzo