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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Kees Cook , Peter Zijlstra , Jann Horn , Pedro Falcato , Hugh Dickins , Pasha Tatashin , Pratyush Yadav , "Matthew Wilcox (Oracle)" , Jan Kara , Zi Yan , Lance Yang , Rik van Riel , Harry Yoo , Alistair Popple , Nico Pache , Ryan Roberts , Dev Jain , Usama Arif , Matthew Brost , John Hubbard , Peter Xu , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Oscar Salvador , Minchan Kim , Muchun Song , Xu Xin , Chengming Zhou , Chris Li , Kemeng Shi , Nhat Pham , Youngjun Park , Gregory Price , Joshua Hahn , Rakie Kim , Ying Huang , Jason Gunthorpe , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-cxl@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH 42/46] mm/huge_memory: split huge PUDs when adjusting VMA boundaries Message-ID: References: <20261027-fixes_core-v1-0-6f74bec9a5bb@kernel.org> <20261027-fixes_core-v1-42-6f74bec9a5bb@kernel.org> X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261027-fixes_core-v1-42-6f74bec9a5bb@kernel.org> On Tue, Oct 27, 2026 at 12:18:02AM +0100, David Hildenbrand (Arm) wrote: > PFN mappings such as VFIO PCI BAR mappings can use PUD leaves while > allowing VMA splits at page granularity. However, vma_adjust_trans_huge() > only handles PMDs. > > After faulting a writable PUD-sized VFIO BAR mapping, changing its first > page to read-only with mprotect() makes the PMD walk treat the PUD leaf > as a PMD table. Applying MADV_DONTFORK to the first PMD-sized chunk leaves > the PUD spanning two VMAs: the PMD-aligned boundary skips splitting. > > Walk the page tables once at each adjusted boundary and split a huge > PUD or PMD leaf as needed. Handle PUD leaves even when the boundary is > PMD-aligned. > > Fixes: f9e54c3a2f5b7 ("vfio/pci: implement huge_fault support") > Cc: stable@vger.kernel.org > Signed-off-by: David Hildenbrand (Arm) > --- > mm/huge_memory.c | 70 ++++++++++++++++++++++++++++++++++++++++++++++++-------- > 1 file changed, 60 insertions(+), 10 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 08e58cda7d37d..d090642ac8c2b 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -3503,16 +3503,66 @@ void split_huge_pmd_address(struct vm_area_struct *vma, unsigned long address, > __split_huge_pmd(vma, pmd, address, freeze); > } > > -static inline void split_huge_pmd_if_needed(struct vm_area_struct *vma, unsigned long address) > +static bool pud_split_required(const struct vm_area_struct *vma, > + unsigned long address) > { > + if (!IS_ENABLED(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)) > + return false; > + if (IS_ALIGNED(address, HPAGE_PUD_SIZE)) > + return false; > /* > - * If the new address isn't hpage aligned and it could previously > - * contain an hugepage: check if we need to split an huge pmd. > + * Only selected VMAs support mapping PUDs for now. Lift the > + * restriction once this changes. > */ > - if (!IS_ALIGNED(address, HPAGE_PMD_SIZE) && > - range_in_vma(vma, ALIGN_DOWN(address, HPAGE_PMD_SIZE), > - ALIGN(address, HPAGE_PMD_SIZE))) > - split_huge_pmd_address(vma, address, false); > + if (!vma_is_dax(vma) && !vma_test_any(vma, VMA_PFNMAP_BIT, VMA_MIXEDMAP_BIT)) > + return false; > + return range_in_vma(vma, ALIGN_DOWN(address, HPAGE_PUD_SIZE), > + ALIGN(address, HPAGE_PUD_SIZE)); > +} > + > +static bool pmd_split_required(const struct vm_area_struct *vma, > + unsigned long address) > +{ > + if (IS_ALIGNED(address, HPAGE_PMD_SIZE)) > + return false; > + > + return range_in_vma(vma, ALIGN_DOWN(address, HPAGE_PMD_SIZE), > + ALIGN(address, HPAGE_PMD_SIZE)); > +} > + > +static void split_huge_leaf(struct vm_area_struct *vma, > + unsigned long address) > +{ > + const bool split_pud = pud_split_required(vma, address); > + const bool split_pmd = pmd_split_required(vma, address); > + pgd_t *pgd; > + p4d_t *p4d; > + pud_t *pud; > + pmd_t *pmd; > + > + /* > + * mmap, VMA and rmap write locks protect the walk; the split helpers > + * take the page table locks. > + */ > + if (!split_pud && !split_pmd) > + return; > + > + pgd = pgd_offset(vma->vm_mm, address); > + if (!pgd_present(*pgd)) > + return; > + > + p4d = p4d_offset(pgd, address); > + if (!p4d_present(*p4d)) > + return; > + > + pud = pud_offset(p4d, address); > + if (split_pud && pud_leaf(*pud)) > + __split_huge_pud(vma, pud, address); > + if (!split_pmd || !pud_present(*pud)) > + return; This looks wrong. We should bail out here if PUD is was a leaf and wasn't split. pmd_offset() below expects page table PUD entry. > + pmd = pmd_offset(pud, address); > + __split_huge_pmd(vma, pmd, address, false); > } > > void vma_adjust_trans_huge(struct vm_area_struct *vma, -- Kiryl Shutsemau / Kirill A. Shutemov