linux-kernel - Re: [PATCH v2 4/5] guest_memfd: add support for userfaultfd minor mode

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Message-ID: <aSigyHj01snbfii7@kernel.org>
Date: Thu, 27 Nov 2025 21:04:40 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Nikita Kalyazin <kalyazin@...zon.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>, Paolo Bonzini <pbonzini@...hat.com>,
	Peter Xu <peterx@...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 v2 4/5] guest_memfd: add support for userfaultfd minor
 mode

On Thu, Nov 27, 2025 at 11:19:35AM +0000, Nikita Kalyazin wrote:
> 
> 
> On 27/11/2025 10:36, Mike Rapoport wrote:
> > On Wed, Nov 26, 2025 at 04:49:31PM +0000, Nikita Kalyazin wrote:
> > > On 25/11/2025 18:38, Mike Rapoport wrote:
> > > > From: "Mike Rapoport (Microsoft)" <rppt@...nel.org>
> > > > 
> > > > +#ifdef CONFIG_USERFAULTFD
> > > > +static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t pgoff)
> > > 
> > > We have to name it differently, otherwise it clashes with the existing one
> > > in this file.
> > 
> > It's all David's fault! ;-P
> > How about kvm_gmem_get_prepared_folio() ?
> 
> I'm afraid it may not be ideal due to preparedness tracking being removed
> from guest_memfd at some point [1].  Would it be too bad to add an
> indication to userfaultfd in the name somehow given that it's already
> guarded by the config?

Hmm, shmem also has this clash. There I picked shmem_get_folio_noalloc()
because that describes well what it does: lookup folio in the page cache,
grab it if it's there or return -ENOENT if it's missing.
That's also what hugetlb does for uffd minor fault.

The guest_memfd implementation I copied from one of the older postings
allocates the folio if it's not in the page cache and it seems to me that
it also should only look up existing folios to keep uffd minor semantics
uniform.
 
Then it makes sense also to name the vm_ops method get_folio_noalloc().

-- 
Sincerely yours,
Mike.