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

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Message-ID: <4405c306-9d7c-4fd6-9ea6-2ed1b73f5c2e@amazon.com>
Date: Fri, 28 Nov 2025 12:15:51 +0000
From: Nikita Kalyazin <kalyazin@...zon.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>, 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 27/11/2025 19:04, Mike Rapoport wrote:
> 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.

I can't see a reason for guest_memfd to deviate from shmem and hugetlb 
here so makes sense to me.

> 
> Then it makes sense also to name the vm_ops method get_folio_noalloc().
> 
> --
> Sincerely yours,
> Mike.