linux-kernel - Re: [PATCH RFC 09/17] userfaultfd: introduce vm_uffd_ops->alloc

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Message-ID: <aYzadvQRCw3iOVsc@x1.local>
Date: Wed, 11 Feb 2026 14:37:26 -0500
From: Peter Xu <peterx@...hat.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: linux-mm@...ck.org, Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	David Hildenbrand <david@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	James Houghton <jthoughton@...gle.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Michal Hocko <mhocko@...e.com>, Muchun Song <muchun.song@...ux.dev>,
	Nikita Kalyazin <kalyazin@...zon.com>,
	Oscar Salvador <osalvador@...e.de>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Sean Christopherson <seanjc@...gle.com>,
	Shuah Khan <shuah@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH RFC 09/17] userfaultfd: introduce
 vm_uffd_ops->alloc_folio()

On Sun, Feb 08, 2026 at 12:22:52PM +0200, Mike Rapoport wrote:
> On Mon, Feb 02, 2026 at 05:13:20PM -0500, Peter Xu wrote:
> > On Tue, Jan 27, 2026 at 09:29:28PM +0200, Mike Rapoport wrote:
> > 
> > [...]
> > 
> > > -static int mfill_atomic_pte_zeroed_folio(pmd_t *dst_pmd,
> > > -					 struct vm_area_struct *dst_vma,
> > > -					 unsigned long dst_addr)
> > > +static int mfill_atomic_pte_copy(struct mfill_state *state)
> > >  {
> > > -	struct folio *folio;
> > > -	int ret = -ENOMEM;
> > > -
> > > -	folio = vma_alloc_zeroed_movable_folio(dst_vma, dst_addr);
> > > -	if (!folio)
> > > -		return ret;
> > > -
> > > -	if (mem_cgroup_charge(folio, dst_vma->vm_mm, GFP_KERNEL))
> > > -		goto out_put;
> > > +	const struct vm_uffd_ops *ops = vma_uffd_ops(state->vma);
> > >  
> > > -	/*
> > > -	 * The memory barrier inside __folio_mark_uptodate makes sure that
> > > -	 * zeroing out the folio become visible before mapping the page
> > > -	 * using set_pte_at(). See do_anonymous_page().
> > > -	 */
> > > -	__folio_mark_uptodate(folio);
> > > +	return __mfill_atomic_pte(state, ops);
> > > +}
> > >  
> > > -	ret = mfill_atomic_install_pte(dst_pmd, dst_vma, dst_addr,
> > > -				       &folio->page, true, 0);
> > > -	if (ret)
> > > -		goto out_put;
> > > +static int mfill_atomic_pte_zeroed_folio(struct mfill_state *state)
> > > +{
> > > +	const struct vm_uffd_ops *ops = vma_uffd_ops(state->vma);
> > >  
> > > -	return 0;
> > > -out_put:
> > > -	folio_put(folio);
> > > -	return ret;
> > > +	return __mfill_atomic_pte(state, ops);
> > >  }
> > >  
> > >  static int mfill_atomic_pte_zeropage(struct mfill_state *state)
> > > @@ -542,7 +546,7 @@ static int mfill_atomic_pte_zeropage(struct mfill_state *state)
> > >  	int ret;
> > >  
> > >  	if (mm_forbids_zeropage(dst_vma->vm_mm))
> > > -		return mfill_atomic_pte_zeroed_folio(dst_pmd, dst_vma, dst_addr);
> > > +		return mfill_atomic_pte_zeroed_folio(state);
> > 
> > After this patch, mfill_atomic_pte_zeroed_folio() should be 100% the same
> > impl with mfill_atomic_pte_copy(), so IIUC we can drop it.
> 
> It will be slightly different after the next patch to emphasize that
> copying into MAP_PRIVATE actually creates anonymous memory.

True.  It might be helpful to leave a line in the commit message so it's
intentional to temporarily have two functions do the same thing, but I'm OK
either way.

Thanks,

-- 
Peter Xu