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Howlett" , Lorenzo Stoakes , Vlastimil Babka , Jann Horn , Joshua Hahn , Rakie Kim , Ying Huang , Peter Xu , Jason Gunthorpe , Sasha Levin Subject: [PATCH 6.18.y 3/3] mm/mempolicy: use vm_normal_folio_pmd() in queue_folios_pmd() Date: Tue, 10 Nov 2026 19:33:14 -0500 Message-ID: <20261111003308.271305-3-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20261111003308.271305-1-sashal@kernel.org> References: <20261107050938-failed-6.18-9057170a941f@kernel.org> <20261111003308.271305-1-sashal@kernel.org> X-Mailing-List: stable@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Gregory Price [ Upstream commit 9057170a941f0c5f4058f6fb9ab5d5fc9fcc9e83 ] Patch series "mm: stop calling pmd_folio() on special PMDs", v3. Two page table walkers resolve the folio behind a PMD with pmd_folio(), which is only valid for a PMD mapping a refcounted struct page: madvise_cold_or_pageout_pte_range() mm/madvise.c queue_folios_pmd() mm/mempolicy.c vmf_insert_pfn_pmd() installs special PMDs holding a raw pfn that need not have a memmap entry at all. Both walkers can reach one and fault on the first folio field read. The PTE halves of both already use vm_normal_folio(); these two patches make the PMD halves match. The four callers of vmf_insert_pfn_pmd(), and which walker each reaches: drivers/vfio/pci/vfio_pci_core.c VM_PFNMAP mempolicy drivers/gpu/drm/drm_gem_shmem_helper.c VM_PFNMAP mempolicy drivers/gpu/drm/panthor/panthor_gem.c VM_PFNMAP mempolicy drivers/hv/mshv_vtl_main.c VM_MIXEDMAP both can_madv_lru_vma() rejects VM_PFNMAP, so only mshv_vtl_low reaches the madvise walker, and that needs CAP_SYS_ADMIN. queue_pages_walk_ops supplies its own ->test_walk, so walk_page_test()'s generic VM_PFNMAP skip never runs and vfio-pci is reachable by any process holding the device fd. Hence the different stable tags. One behaviour change: mbind(MPOL_MF_STRICT) over a PMD mapped VM_PFNMAP region now returns 0 rather than -EIO. The PTE loop already returned 0 there. drm_gem_shmem and panthor are where this is observable, since they PMD map pages that do have a memmap entry and so never faulted. Reproducer ========== No hardware needed. An out of tree module stands in for the drivers above: three misc devices, each with a ->huge_fault calling vmf_insert_pfn_pmd(), plus VM_HUGEPAGE so the fault path takes the PMD branch. /dev/pmdspec_mixed VM_MIXEDMAP, pfn at the 1 TiB mark, no memmap /dev/pmdspec_pfnmap VM_PFNMAP, pfn at the 1 TiB mark, no memmap /dev/pmdspec_real VM_PFNMAP, real alloc_pages(PMD_ORDER) on node 0 Userspace maps the device into a PMD aligned window, reads one byte to fault the PMD in, checks a module parameter to confirm it went in, then issues the operation. vng --run --user root --memory 4G --verbose \ --append "numa=fake=2" \ --exec "insmod pmdspec.ko && ./pmdspec_test " numa=fake=2 gives a node 1 to bind to; the module allocates its real page on node 0, which is what makes queue_folio_required() true. subtest operation parent series -------------------------------------------------------------------- madv_cold madvise(MADV_COLD) oops ret=0 madv_pageout madvise(MADV_PAGEOUT) oops ret=0 mbind_mixed mbind(MPOL_BIND, n1, MPOL_MF_MOVE) oops ret=0 mbind_pfnmap mbind(MPOL_BIND, n1, MPOL_MF_STRICT) oops ret=0 mbind_real mbind(MPOL_BIND, n1, MPOL_MF_STRICT) -EIO ret=0 Two things the table shows that are easy to miss in the code: - mbind_mixed passes only MPOL_MF_MOVE. MPOL_MF_STRICT is not needed for a VM_MIXEDMAP vma: walk_page_test() only skips VM_PFNMAP, and vma_migratable() is true for VM_MIXEDMAP. - mbind_real demonstrates the user visible change (-EIO -> 0) This patch (of 2): mmap a VM_PFNMAP region whose ->huge_fault installs a PMD through vmf_insert_pfn_pmd() - a vfio-pci MMIO BAR does this - then mbind(p, len, MPOL_BIND, &mask, maxnode, MPOL_MF_STRICT); With a stand-in module for the driver: BUG: unable to handle page fault for address: fffff96dc0000008 RIP: 0010:queue_folios_pte_range+0xaf/0x440 walk_pgd_range+0x52b/0xaf0 __walk_page_range+0x6a/0x1d0 walk_page_range_mm_unsafe+0x193/0x230 queue_pages_range+0x64/0xa0 do_mbind+0x25e/0x640 queue_folios_pmd(), inlined above, calls pmd_folio() on that PMD. The pfn is raw MMIO with no memmap entry, so the folio lands in unpopulated vmemmap. Neither guard stops the walk: walk_page_test() skips VM_PFNMAP, but queue_pages_walk_ops supplies ->test_walk, so it never runs queue_pages_test_walk() honours vma_migratable(), but only while MPOL_MF_STRICT is clear A VM_MIXEDMAP vma needs neither flag, being vma_migratable(), so plain mbind(MPOL_MF_MOVE) reaches this too - and there the bad folio carries on into migrate_folio_add() and folio_isolate_lru(). mshv_vtl_low is such a mapping. Use vm_normal_folio_pmd() and skip on NULL, as the PTE loop in queue_folios_pte_range() already does with vm_normal_folio(). This also filters the huge zero PMD, so its separate check is no longer needed. mbind(MPOL_MF_STRICT) over a PMD mapped VM_PFNMAP region now returns 0 rather than -EIO. The PTE loop already returned 0 there. Link: https://lore.kernel.org/20260926105110.2156652-1-gourry@gourry.net Link: https://lore.kernel.org/20260926105110.2156652-2-gourry@gourry.net Fixes: 3c8e44c9b369 ("mm: mark special bits for huge pfn mappings when inject") Signed-off-by: Gregory Price (Meta) Signed-off-by: Andrew Morton Reported-by: sashiko-bot Closes: https://sashiko.dev/#/patchset/20260817220810.1175596-1-gourry%40gourry.net Acked-by: David Hildenbrand (Arm) Reviewed-by: Zi Yan Assisted-by: LLM Cc: Liam R. Howlett Cc: Lorenzo Stoakes Cc: Vlastimil Babka Cc: Jann Horn Cc: Joshua Hahn Cc: Rakie Kim Cc: Ying Huang Cc: Peter Xu Cc: Jason Gunthorpe Cc: Signed-off-by: Sasha Levin --- mm/mempolicy.c | 13 +++++-------- 1 file changed, 5 insertions(+), 8 deletions(-) diff --git a/mm/mempolicy.c b/mm/mempolicy.c index a6ee9526dc25e..4ceffb524c57b 100644 --- a/mm/mempolicy.c +++ b/mm/mempolicy.c @@ -646,7 +646,8 @@ static inline bool queue_folio_required(struct folio *folio, return node_isset(nid, *qp->nmask) == !(flags & MPOL_MF_INVERT); } -static void queue_folios_pmd(pmd_t *pmd, struct mm_walk *walk) +static void queue_folios_pmd(pmd_t *pmd, unsigned long addr, + struct mm_walk *walk) { struct folio *folio; struct queue_pages *qp = walk->private; @@ -657,13 +658,9 @@ static void queue_folios_pmd(pmd_t *pmd, struct mm_walk *walk) qp->nr_failed++; return; } - folio = pmd_folio(pmdval); - if (folio_is_zone_device(folio)) + folio = vm_normal_folio_pmd(walk->vma, addr, pmdval); + if (!folio || folio_is_zone_device(folio)) return; - if (is_huge_zero_folio(folio)) { - walk->action = ACTION_CONTINUE; - return; - } if (!queue_folio_required(folio, qp)) return; if (!(qp->flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) || @@ -696,7 +693,7 @@ static int queue_folios_pte_range(pmd_t *pmd, unsigned long addr, ptl = pmd_trans_huge_lock(pmd, vma); if (ptl) { - queue_folios_pmd(pmd, walk); + queue_folios_pmd(pmd, addr, walk); spin_unlock(ptl); goto out; } -- 2.53.0