linux-kernel - Re: [PATCH RFC 04/17] userfaultfd: introduce mfill_get_vma() and mfill_put

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Message-ID: <aYhdZ_fOrkmAsSEX@kernel.org>
Date: Sun, 8 Feb 2026 11:54:47 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Peter Xu <peterx@...hat.com>
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 04/17] userfaultfd: introduce mfill_get_vma() and
 mfill_put_vma()

Hi Peter,

On Mon, Feb 02, 2026 at 04:49:09PM -0500, Peter Xu wrote:
> Hi, Mike,
> 
> On Tue, Jan 27, 2026 at 09:29:23PM +0200, Mike Rapoport wrote:
> > From: "Mike Rapoport (Microsoft)" <rppt@...nel.org>
> > 
> > Split the code that finds, locks and verifies VMA from mfill_atomic()
> > into a helper function.
> > 
> > This function will be used later during refactoring of
> > mfill_atomic_pte_copy().
> > 
> > Add a counterpart mfill_put_vma() helper that unlocks the VMA and
> > releases map_changing_lock.
> > 
> > Signed-off-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> > ---
> >  mm/userfaultfd.c | 124 ++++++++++++++++++++++++++++-------------------
> >  1 file changed, 73 insertions(+), 51 deletions(-)
> > 
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index 9dd285b13f3b..45d8f04aaf4f 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -157,6 +157,73 @@ static void uffd_mfill_unlock(struct vm_area_struct *vma)
> >  }
> >  #endif
> >  
> > +static void mfill_put_vma(struct mfill_state *state)
> > +{
> > +	up_read(&state->ctx->map_changing_lock);
> > +	uffd_mfill_unlock(state->vma);
> > +	state->vma = NULL;
> > +}
> > +
> > +static int mfill_get_vma(struct mfill_state *state)
> > +{
> > +	struct userfaultfd_ctx *ctx = state->ctx;
> > +	uffd_flags_t flags = state->flags;
> > +	struct vm_area_struct *dst_vma;
> > +	int err;
> > +
> > +	/*
> > +	 * Make sure the vma is not shared, that the dst range is
> > +	 * both valid and fully within a single existing vma.
> > +	 */
> > +	dst_vma = uffd_mfill_lock(ctx->mm, state->dst_start, state->len);
> > +	if (IS_ERR(dst_vma))
> > +		return PTR_ERR(dst_vma);
> > +
> > +	/*
> > +	 * If memory mappings are changing because of non-cooperative
> > +	 * operation (e.g. mremap) running in parallel, bail out and
> > +	 * request the user to retry later
> > +	 */
> > +	down_read(&ctx->map_changing_lock);
> > +	err = -EAGAIN;
> > +	if (atomic_read(&ctx->mmap_changing))
> > +		goto out_unlock;
> > +
> > +	err = -EINVAL;
> > +
> > +	/*
> > +	 * shmem_zero_setup is invoked in mmap for MAP_ANONYMOUS|MAP_SHARED but
> > +	 * it will overwrite vm_ops, so vma_is_anonymous must return false.
> > +	 */
> > +	if (WARN_ON_ONCE(vma_is_anonymous(dst_vma) &&
> > +	    dst_vma->vm_flags & VM_SHARED))
> > +		goto out_unlock;
> > +
> > +	/*
> > +	 * validate 'mode' now that we know the dst_vma: don't allow
> > +	 * a wrprotect copy if the userfaultfd didn't register as WP.
> > +	 */
> > +	if ((flags & MFILL_ATOMIC_WP) && !(dst_vma->vm_flags & VM_UFFD_WP))
> > +		goto out_unlock;
> > +
> > +	if (is_vm_hugetlb_page(dst_vma))
> > +		goto out;
> > +
> > +	if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
> > +		goto out_unlock;
> > +	if (!vma_is_shmem(dst_vma) &&
> > +	    uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
> > +		goto out_unlock;
> 
> IMHO it's a bit weird to check for vma permissions in a get_vma() function.
> 
> Also, in the follow up patch it'll be also reused in
> mfill_copy_folio_retry() which doesn't need to check vma permission.
> 
> Maybe we can introduce mfill_vma_check() for these two checks? Then we can
> also drop the slightly weird is_vm_hugetlb_page() check (and "out" label)
> above.

This version of get_vma() keeps the checks exactly as they were when we
were retrying after dropping the lock and I prefer to have them this way to
begin with.
Later we can optimize this further after the dust settles after these
changes.

-- 
Sincerely yours,
Mike.