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Date: Thu, 26 Mar 2026 17:26:34 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: James Houghton <jthoughton@google.com>
Cc: Mike Rapoport <rppt@kernel.org>, Andrea Arcangeli <aarcange@redhat.com>,
 Axel Rasmussen <axelrasmussen@google.com>, Baolin Wang
 <baolin.wang@linux.alibaba.com>, David Hildenbrand <david@kernel.org>, Hugh
 Dickins <hughd@google.com>, "Liam R. Howlett" <Liam.Howlett@oracle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@oracle.com>, "Matthew Wilcox (Oracle)"
 <willy@infradead.org>, Michal Hocko <mhocko@suse.com>, Muchun Song
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 <shuah@kernel.org>, Suren Baghdasaryan <surenb@google.com>, Vlastimil Babka
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 linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
 linux-mm@kvack.org
Subject: Re: [PATCH v2 08/15] shmem, userfaultfd: use a VMA callback to
 handle UFFDIO_CONTINUE
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On Thu, 26 Mar 2026 16:43:27 -0700 James Houghton <jthoughton@google.com> wrote:

> On Fri, Mar 6, 2026 at 9:19 AM Mike Rapoport <rppt@kernel.org> wrote:
> >
> > From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
> >
> > When userspace resolves a page fault in a shmem VMA with UFFDIO_CONTINUE
> > it needs to get a folio that already exists in the pagecache backing
> > that VMA.
> >
> > Instead of using shmem_get_folio() for that, add a get_folio_noalloc()
> > method to 'struct vm_uffd_ops' that will return a folio if it exists in
> > the VMA's pagecache at given pgoff.
> >
> > Implement get_folio_noalloc() method for shmem and slightly refactor
> > userfaultfd's mfill_get_vma() and mfill_atomic_pte_continue() to support
> > this new API.
> >
> > Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> 
> I found myself asking why HugeTLB wasn't included here; it's because
> we call into hugetlb_mfill_atomic_pte() pretty early. It would be nice
> to have HugeTLB implement get_folio_noalloc() and use the same path as
> shmem, but we don't have to do that now.
> 
> Feel free to add:
> 
> Reviewed-by: James Houghton <jthoughton@google.com>

Thanks.

> >  static int mfill_atomic_pte_continue(struct mfill_state *state)
> >  {
> >         struct vm_area_struct *dst_vma = state->vma;
> > +       const struct vm_uffd_ops *ops = vma_uffd_ops(dst_vma);
> >         unsigned long dst_addr = state->dst_addr;
> >         pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);
> >         struct inode *inode = file_inode(dst_vma->vm_file);
> > @@ -586,16 +590,13 @@ static int mfill_atomic_pte_continue(struct mfill_state *state)
> >         struct page *page;
> >         int ret;
> >
> > -       ret = shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC);
> > +       if (!ops)
> > +               return -EOPNOTSUPP;
> 
> Perhaps WARN_ON_ONCE(!ops)?
> 

fyi, I (and I expect many others) tend to consider a "Reviewed-by:" as
end-of-message, So if you have additional comments then add something
like "comments below" so we know to keep scrolling!

If there are no additional comments, simply zap all quoted material after the
R-b to remove all doubt.