linux-kernel - Re: [PATCH 03/16] mm: add vma_desc_size(), vma_desc

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Message-ID: <090675bd-cb18-4148-967b-52cca452e07b@lucifer.local>
Date: Mon, 8 Sep 2025 15:09:43 +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 03/16] mm: add vma_desc_size(), vma_desc_pages() helpers

On Mon, Sep 08, 2025 at 10:32:24AM -0300, Jason Gunthorpe wrote:
> On Mon, Sep 08, 2025 at 02:12:00PM +0100, Lorenzo Stoakes wrote:
> > On Mon, Sep 08, 2025 at 09:51:01AM -0300, Jason Gunthorpe wrote:
> > > On Mon, Sep 08, 2025 at 12:10:34PM +0100, Lorenzo Stoakes wrote:
> > > >  static int secretmem_mmap_prepare(struct vm_area_desc *desc)
> > > >  {
> > > > -	const unsigned long len = desc->end - desc->start;
> > > > +	const unsigned long len = vma_desc_size(desc);
> > > >
> > > >  	if ((desc->vm_flags & (VM_SHARED | VM_MAYSHARE)) == 0)
> > > >  		return -EINVAL;
> > >
> > > I wonder if we should have some helper for this shared check too, it
> > > is a bit tricky with the two flags. Forced-shared checks are pretty
> > > common.
> >
> > Sure can add.
> >
> > >
> > > vma_desc_must_be_shared(desc) ?
> >
> > Maybe _could_be_shared()?
>
> It is not could, it is must.

I mean VM_MAYSHARE is a nonsense anyway, but _in theory_ VM_MAYSHARE &&
!VM_SHARE means we _could_ share it.

But in reality of course this isn't a real thing.

Perhaps vma_desc_is_shared() or something, I obviously don't want to get stuck
on semantics here :) [he says, while getting obviously stuck on semantics] :P

>
> Perhaps
>
> !vma_desc_cowable()
>
> Is what many drivers are really trying to assert.

Well no, because:

static inline bool is_cow_mapping(vm_flags_t flags)
{
	return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
}

Read-only means !CoW.

Hey we've made a rod for own backs! Again!

>
> Jason

Cheers, Lorenzo