linux-kernel - Re: [PATCH v2 3/5] mm: introduce VM_FAULT_UFFD

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Message-ID: <de66cef2-de14-4503-bbc6-21467e0e3e29@kernel.org>
Date: Wed, 26 Nov 2025 11:19:30 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Mike Rapoport <rppt@...nel.org>, linux-mm@...ck.org
Cc: Andrea Arcangeli <aarcange@...hat.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Axel Rasmussen <axelrasmussen@...gle.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.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>, Nikita Kalyazin <kalyazin@...zon.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 3/5] mm: introduce VM_FAULT_UFFD_MINOR fault reason

> @@ -1564,7 +1571,8 @@ enum vm_fault_reason {
>   	{ VM_FAULT_FALLBACK,            "FALLBACK" },	\
>   	{ VM_FAULT_DONE_COW,            "DONE_COW" },	\
>   	{ VM_FAULT_NEEDDSYNC,           "NEEDDSYNC" },	\
> -	{ VM_FAULT_COMPLETED,           "COMPLETED" }
> +	{ VM_FAULT_COMPLETED,           "COMPLETED" },	\
> +	{ VM_FAULT_UFFD_MINOR,		"UFFD_MINOR" },	\
>   
>   struct vm_special_mapping {
>   	const char *name;	/* The name, e.g. "[vdso]". */
> diff --git a/mm/memory.c b/mm/memory.c
> index b59ae7ce42eb..94acbac8cefb 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5279,6 +5279,8 @@ static vm_fault_t __do_fault(struct vm_fault *vmf)
>   	}
>   
>   	ret = vma->vm_ops->fault(vmf);
> +	if (unlikely(ret & VM_FAULT_UFFD_MINOR))
> +		return handle_userfault(vmf, VM_UFFD_MINOR);
>   	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY |
>   			    VM_FAULT_DONE_COW)))

If we want to reduce the overhead on the fast path, we can simply do

if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY |
		    VM_FAULT_DONE_COW | VM_FAULT_UFFD_MINOR))) {
	if (unlikely(ret & VM_FAULT_UFFD_MINOR))
		return handle_userfault(vmf, VM_UFFD_MINOR);
	return ret;
}

Maybe the compiler already does that to improve the likely case.

LGTM

-- 
Cheers

David