[PATCH v2] drm/shmem-helper: Fix Map huge page mapping in fault handler

[PATCH v2] drm/shmem-helper: Fix Map huge page mapping in fault handler

[Pedro Demarchi Gomes]

When running ./tools/testing/selftests/mm/split_huge_page_test multiple
times with /sys/kernel/mm/transparent_hugepage/shmem_enabled and
/sys/kernel/mm/transparent_hugepage/enabled set as always the following BUG
occurs:

[  232.728858] ------------[ cut here ]------------
[  232.729458] kernel BUG at mm/memory.c:2276!
[  232.729726] Oops: invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN PTI
[  232.730217] CPU: 19 UID: 60578 PID: 1497 Comm: llvmpipe-9 Not tainted 7.0.0-rc1mm-new+ #19 PREEMPT(lazy)
[  232.730855] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-9.fc43 06/10/2025
[  232.731360] RIP: 0010:walk_to_pmd+0x29e/0x3c0
[  232.731569] Code: d8 5b 5d 41 5c 41 5d 41 5e 41 5f c3 cc cc cc cc 48 89 ea 48 89 de 4c 89 f7 e8 ae 85 ff ff 85 c0 0f 84 1f fe ff ff 31 db eb d0 <0f> 0b 48 89 ea 48 89 de 4c 89 f7 e8 92 8b ff ff 85 c0 75 e8 48 b8
[  232.732614] RSP: 0000:ffff8881aa6ff9a8 EFLAGS: 00010282
[  232.732991] RAX: 8000000142e002e7 RBX: ffff8881433cae10 RCX: dffffc0000000000
[  232.733362] RDX: 0000000000000000 RSI: 00007fb47840b000 RDI: 8000000142e002e7
[  232.733801] RBP: 00007fb47840b000 R08: 0000000000000000 R09: 1ffff110354dff46
[  232.734168] R10: fffffbfff0cb921d R11: 00000000910da5ce R12: 1ffffffff0c1fcdd
[  232.734459] R13: 1ffffffff0c23f36 R14: ffff888171628040 R15: 0000000000000000
[  232.734861] FS:  00007fb4907f86c0(0000) GS:ffff888791f2c000(0000) knlGS:0000000000000000
[  232.735265] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  232.735548] CR2: 00007fb47840be00 CR3: 000000015e6dc000 CR4: 00000000000006f0
[  232.736031] Call Trace:
[  232.736273]  <TASK>
[  232.736500]  get_locked_pte+0x1f/0xa0
[  232.736878]  insert_pfn+0x9f/0x350
[  232.737190]  ? __pfx_pat_pagerange_is_ram+0x10/0x10
[  232.737614]  ? __pfx_insert_pfn+0x10/0x10
[  232.737990]  ? __pfx_css_rstat_updated+0x10/0x10
[  232.738281]  ? __pfx_pfn_modify_allowed+0x10/0x10
[  232.738552]  ? lookup_memtype+0x62/0x180
[  232.738761]  vmf_insert_pfn_prot+0x14b/0x340
[  232.739012]  ? __pfx_vmf_insert_pfn_prot+0x10/0x10
[  232.739247]  ? __pfx___might_resched+0x10/0x10
[  232.739475]  drm_gem_shmem_fault.cold+0x18/0x39
[  232.739677]  ? rcu_read_unlock+0x20/0x70
[  232.739882]  __do_fault+0x251/0x7b0
[  232.740028]  do_fault+0x6e1/0xc00
[  232.740167]  ? __lock_acquire+0x590/0xc40
[  232.740335]  handle_pte_fault+0x439/0x760
[  232.740498]  ? mtree_range_walk+0x252/0xae0
[  232.740669]  ? __pfx_handle_pte_fault+0x10/0x10
[  232.740899]  __handle_mm_fault+0xa02/0xf30
[  232.741066]  ? __pfx___handle_mm_fault+0x10/0x10
[  232.741255]  ? find_vma+0xa1/0x120
[  232.741403]  handle_mm_fault+0x2bf/0x8f0
[  232.741564]  do_user_addr_fault+0x2d3/0xed0
[  232.741736]  ? trace_page_fault_user+0x1bf/0x240
[  232.741969]  exc_page_fault+0x87/0x120
[  232.742124]  asm_exc_page_fault+0x26/0x30
[  232.742288] RIP: 0033:0x7fb4d73ed546
[  232.742441] Code: 66 41 0f 6f fb 66 44 0f 6d dc 66 44 0f 6f c6 66 41 0f 6d f1 66 0f 6c fc 66 45 0f 6c c1 66 44 0f 6f c9 66 0f 6d ca 66 0f db f0 <66> 0f df 04 08 66 44 0f 6c ca 66 45 0f db c2 66 44 0f df 10 66 44
[  232.743193] RSP: 002b:00007fb4907f68a0 EFLAGS: 00010206
[  232.743565] RAX: 00007fb47840aa00 RBX: 00007fb4d73ec070 RCX: 0000000000001400
[  232.743871] RDX: 0000000000002800 RSI: 0000000000003c00 RDI: 0000000000000001
[  232.744150] RBP: 0000000000000004 R08: 0000000000001400 R09: 00007fb4d73ec060
[  232.744433] R10: 000055f0261a4288 R11: 00007fb4c013da40 R12: 0000000000000008
[  232.744712] R13: 0000000000000000 R14: 4332322132212110 R15: 0000000000000004
[  232.746616]  </TASK>
[  232.746711] Modules linked in: nft_nat nft_masq veth bridge stp llc snd_seq_dummy snd_hrtimer snd_seq snd_seq_device snd_timer snd soundcore overlay rfkill nf_conntrack_netbios_ns nf_conntrack_broadcast nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 nf_tables qrtr ppdev 9pnet_virtio 9pnet parport_pc i2c_piix4 netfs pcspkr parport i2c_smbus joydev sunrpc vfat fat loop dm_multipath nfnetlink vsock_loopback vmw_vsock_virtio_transport_common vmw_vsock_vmci_transport zram lz4hc_compress vmw_vmci lz4_compress vsock e1000 bochs serio_raw ata_generic pata_acpi scsi_dh_rdac scsi_dh_emc scsi_dh_alua i2c_dev fuse qemu_fw_cfg
[  232.749308] ---[ end trace 0000000000000000 ]---
[  232.749507] RIP: 0010:walk_to_pmd+0x29e/0x3c0
[  232.749692] Code: d8 5b 5d 41 5c 41 5d 41 5e 41 5f c3 cc cc cc cc 48 89 ea 48 89 de 4c 89 f7 e8 ae 85 ff ff 85 c0 0f 84 1f fe ff ff 31 db eb d0 <0f> 0b 48 89 ea 48 89 de 4c 89 f7 e8 92 8b ff ff 85 c0 75 e8 48 b8
[  232.750428] RSP: 0000:ffff8881aa6ff9a8 EFLAGS: 00010282
[  232.750645] RAX: 8000000142e002e7 RBX: ffff8881433cae10 RCX: dffffc0000000000
[  232.750954] RDX: 0000000000000000 RSI: 00007fb47840b000 RDI: 8000000142e002e7
[  232.751232] RBP: 00007fb47840b000 R08: 0000000000000000 R09: 1ffff110354dff46
[  232.751514] R10: fffffbfff0cb921d R11: 00000000910da5ce R12: 1ffffffff0c1fcdd
[  232.751837] R13: 1ffffffff0c23f36 R14: ffff888171628040 R15: 0000000000000000
[  232.752124] FS:  00007fb4907f86c0(0000) GS:ffff888791f2c000(0000) knlGS:0000000000000000
[  232.752441] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  232.752674] CR2: 00007fb47840be00 CR3: 000000015e6dc000 CR4: 00000000000006f0
[  232.752983] Kernel panic - not syncing: Fatal exception
[  232.753510] Kernel Offset: disabled
[  232.754643] ---[ end Kernel panic - not syncing: Fatal exception ]---

This happens when two concurrent page faults occur within the same PMD range.
One fault installs a PMD mapping through vmf_insert_pfn_pmd(), while the other
attempts to install a PTE mapping via vmf_insert_pfn(). The bug is
triggered because a pmd_trans_huge is not expected when walking the page
table inside vmf_insert_pfn.

Avoid this race by adding a huge_fault callback to drm_gem_shmem_vm_ops so that
PMD-sized mappings are handled through the appropriate huge page fault path.

Fixes: 211b9a39f261 ("drm/shmem-helper: Map huge pages in fault handler")
Signed-off-by: Pedro Demarchi Gomes <pedrodemargomes@gmail.com>
---

Changes in v2:
 - Keep the #ifdef unindented
 - Create drm_gem_shmem_any_fault to handle faults of any order and use
drm_gem_shmem_[huge_]fault() as wrappers

---
 drivers/gpu/drm/drm_gem_shmem_helper.c | 67 +++++++++++++++-----------
 1 file changed, 39 insertions(+), 28 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index 7b5a49935ae4..fb8b74ac3057 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -550,36 +550,19 @@ int drm_gem_shmem_dumb_create(struct drm_file *file, struct drm_device *dev,
 }
 EXPORT_SYMBOL_GPL(drm_gem_shmem_dumb_create);
 
-static bool drm_gem_shmem_try_map_pmd(struct vm_fault *vmf, unsigned long addr,
-				      struct page *page)
-{
-#ifdef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP
-	unsigned long pfn = page_to_pfn(page);
-	unsigned long paddr = pfn << PAGE_SHIFT;
-	bool aligned = (addr & ~PMD_MASK) == (paddr & ~PMD_MASK);
-
-	if (aligned &&
-	    pmd_none(*vmf->pmd) &&
-	    folio_test_pmd_mappable(page_folio(page))) {
-		pfn &= PMD_MASK >> PAGE_SHIFT;
-		if (vmf_insert_pfn_pmd(vmf, pfn, false) == VM_FAULT_NOPAGE)
-			return true;
-	}
-#endif
-
-	return false;
-}
-
-static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
+static vm_fault_t drm_gem_shmem_any_fault(struct vm_fault *vmf,
+					   unsigned int order)
 {
 	struct vm_area_struct *vma = vmf->vma;
 	struct drm_gem_object *obj = vma->vm_private_data;
 	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
 	loff_t num_pages = obj->size >> PAGE_SHIFT;
-	vm_fault_t ret;
+	vm_fault_t ret = VM_FAULT_FALLBACK;
 	struct page **pages = shmem->pages;
 	pgoff_t page_offset;
 	unsigned long pfn;
+	unsigned long paddr;
+	bool aligned;
 
 	/* Offset to faulty address in the VMA. */
 	page_offset = vmf->pgoff - vma->vm_pgoff;
@@ -593,13 +576,24 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
 		goto out;
 	}
 
-	if (drm_gem_shmem_try_map_pmd(vmf, vmf->address, pages[page_offset])) {
-		ret = VM_FAULT_NOPAGE;
-		goto out;
-	}
-
 	pfn = page_to_pfn(pages[page_offset]);
-	ret = vmf_insert_pfn(vma, vmf->address, pfn);
+	switch (order) {
+	case 0:
+		ret = vmf_insert_pfn(vma, vmf->address, pfn);
+		break;
+#ifdef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP
+	case PMD_ORDER:
+		paddr = pfn << PAGE_SHIFT;
+		aligned = (vmf->address & ~PMD_MASK) == (paddr & ~PMD_MASK);
+
+		if (aligned &&
+		    folio_test_pmd_mappable(page_folio(pages[page_offset]))) {
+			pfn &= PMD_MASK >> PAGE_SHIFT;
+			ret = vmf_insert_pfn_pmd(vmf, pfn, false);
+		}
+		break;
+#endif
+	}
 
  out:
 	dma_resv_unlock(shmem->base.resv);
@@ -607,6 +601,20 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
 	return ret;
 }
 
+static vm_fault_t drm_gem_shmem_huge_fault(struct vm_fault *vmf,
+					   unsigned int order)
+{
+	if (order != PMD_ORDER)
+		return VM_FAULT_FALLBACK;
+
+	return drm_gem_shmem_any_fault(vmf, order);
+}
+
+static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
+{
+	return drm_gem_shmem_any_fault(vmf, 0);
+}
+
 static void drm_gem_shmem_vm_open(struct vm_area_struct *vma)
 {
 	struct drm_gem_object *obj = vma->vm_private_data;
@@ -643,6 +651,9 @@ static void drm_gem_shmem_vm_close(struct vm_area_struct *vma)
 
 const struct vm_operations_struct drm_gem_shmem_vm_ops = {
 	.fault = drm_gem_shmem_fault,
+#ifdef CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP
+	.huge_fault = drm_gem_shmem_huge_fault,
+#endif
 	.open = drm_gem_shmem_vm_open,
 	.close = drm_gem_shmem_vm_close,
 };
-- 
2.53.0

Re: [PATCH v2] drm/shmem-helper: Fix Map huge page mapping in fault handler

[Boris Brezillon]

On Fri, 13 Mar 2026 11:17:19 -0300
Pedro Demarchi Gomes <pedrodemargomes@gmail.com> wrote:

> When running ./tools/testing/selftests/mm/split_huge_page_test multiple
> times with /sys/kernel/mm/transparent_hugepage/shmem_enabled and
> /sys/kernel/mm/transparent_hugepage/enabled set as always the following BUG
> occurs:
> 
> [  232.728858] ------------[ cut here ]------------
> [  232.729458] kernel BUG at mm/memory.c:2276!
> [  232.729726] Oops: invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN PTI
> [  232.730217] CPU: 19 UID: 60578 PID: 1497 Comm: llvmpipe-9 Not tainted 7.0.0-rc1mm-new+ #19 PREEMPT(lazy)
> [  232.730855] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-9.fc43 06/10/2025
> [  232.731360] RIP: 0010:walk_to_pmd+0x29e/0x3c0
> [  232.731569] Code: d8 5b 5d 41 5c 41 5d 41 5e 41 5f c3 cc cc cc cc 48 89 ea 48 89 de 4c 89 f7 e8 ae 85 ff ff 85 c0 0f 84 1f fe ff ff 31 db eb d0 <0f> 0b 48 89 ea 48 89 de 4c 89 f7 e8 92 8b ff ff 85 c0 75 e8 48 b8
> [  232.732614] RSP: 0000:ffff8881aa6ff9a8 EFLAGS: 00010282
> [  232.732991] RAX: 8000000142e002e7 RBX: ffff8881433cae10 RCX: dffffc0000000000
> [  232.733362] RDX: 0000000000000000 RSI: 00007fb47840b000 RDI: 8000000142e002e7
> [  232.733801] RBP: 00007fb47840b000 R08: 0000000000000000 R09: 1ffff110354dff46
> [  232.734168] R10: fffffbfff0cb921d R11: 00000000910da5ce R12: 1ffffffff0c1fcdd
> [  232.734459] R13: 1ffffffff0c23f36 R14: ffff888171628040 R15: 0000000000000000
> [  232.734861] FS:  00007fb4907f86c0(0000) GS:ffff888791f2c000(0000) knlGS:0000000000000000
> [  232.735265] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [  232.735548] CR2: 00007fb47840be00 CR3: 000000015e6dc000 CR4: 00000000000006f0
> [  232.736031] Call Trace:
> [  232.736273]  <TASK>
> [  232.736500]  get_locked_pte+0x1f/0xa0
> [  232.736878]  insert_pfn+0x9f/0x350
> [  232.737190]  ? __pfx_pat_pagerange_is_ram+0x10/0x10
> [  232.737614]  ? __pfx_insert_pfn+0x10/0x10
> [  232.737990]  ? __pfx_css_rstat_updated+0x10/0x10
> [  232.738281]  ? __pfx_pfn_modify_allowed+0x10/0x10
> [  232.738552]  ? lookup_memtype+0x62/0x180
> [  232.738761]  vmf_insert_pfn_prot+0x14b/0x340
> [  232.739012]  ? __pfx_vmf_insert_pfn_prot+0x10/0x10
> [  232.739247]  ? __pfx___might_resched+0x10/0x10
> [  232.739475]  drm_gem_shmem_fault.cold+0x18/0x39
> [  232.739677]  ? rcu_read_unlock+0x20/0x70
> [  232.739882]  __do_fault+0x251/0x7b0
> [  232.740028]  do_fault+0x6e1/0xc00
> [  232.740167]  ? __lock_acquire+0x590/0xc40
> [  232.740335]  handle_pte_fault+0x439/0x760
> [  232.740498]  ? mtree_range_walk+0x252/0xae0
> [  232.740669]  ? __pfx_handle_pte_fault+0x10/0x10
> [  232.740899]  __handle_mm_fault+0xa02/0xf30
> [  232.741066]  ? __pfx___handle_mm_fault+0x10/0x10
> [  232.741255]  ? find_vma+0xa1/0x120
> [  232.741403]  handle_mm_fault+0x2bf/0x8f0
> [  232.741564]  do_user_addr_fault+0x2d3/0xed0
> [  232.741736]  ? trace_page_fault_user+0x1bf/0x240
> [  232.741969]  exc_page_fault+0x87/0x120
> [  232.742124]  asm_exc_page_fault+0x26/0x30
> [  232.742288] RIP: 0033:0x7fb4d73ed546
> [  232.742441] Code: 66 41 0f 6f fb 66 44 0f 6d dc 66 44 0f 6f c6 66 41 0f 6d f1 66 0f 6c fc 66 45 0f 6c c1 66 44 0f 6f c9 66 0f 6d ca 66 0f db f0 <66> 0f df 04 08 66 44 0f 6c ca 66 45 0f db c2 66 44 0f df 10 66 44
> [  232.743193] RSP: 002b:00007fb4907f68a0 EFLAGS: 00010206
> [  232.743565] RAX: 00007fb47840aa00 RBX: 00007fb4d73ec070 RCX: 0000000000001400
> [  232.743871] RDX: 0000000000002800 RSI: 0000000000003c00 RDI: 0000000000000001
> [  232.744150] RBP: 0000000000000004 R08: 0000000000001400 R09: 00007fb4d73ec060
> [  232.744433] R10: 000055f0261a4288 R11: 00007fb4c013da40 R12: 0000000000000008
> [  232.744712] R13: 0000000000000000 R14: 4332322132212110 R15: 0000000000000004
> [  232.746616]  </TASK>
> [  232.746711] Modules linked in: nft_nat nft_masq veth bridge stp llc snd_seq_dummy snd_hrtimer snd_seq snd_seq_device snd_timer snd soundcore overlay rfkill nf_conntrack_netbios_ns nf_conntrack_broadcast nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 nf_tables qrtr ppdev 9pnet_virtio 9pnet parport_pc i2c_piix4 netfs pcspkr parport i2c_smbus joydev sunrpc vfat fat loop dm_multipath nfnetlink vsock_loopback vmw_vsock_virtio_transport_common vmw_vsock_vmci_transport zram lz4hc_compress vmw_vmci lz4_compress vsock e1000 bochs serio_raw ata_generic pata_acpi scsi_dh_rdac scsi_dh_emc scsi_dh_alua i2c_dev fuse qemu_fw_cfg
> [  232.749308] ---[ end trace 0000000000000000 ]---
> [  232.749507] RIP: 0010:walk_to_pmd+0x29e/0x3c0
> [  232.749692] Code: d8 5b 5d 41 5c 41 5d 41 5e 41 5f c3 cc cc cc cc 48 89 ea 48 89 de 4c 89 f7 e8 ae 85 ff ff 85 c0 0f 84 1f fe ff ff 31 db eb d0 <0f> 0b 48 89 ea 48 89 de 4c 89 f7 e8 92 8b ff ff 85 c0 75 e8 48 b8
> [  232.750428] RSP: 0000:ffff8881aa6ff9a8 EFLAGS: 00010282
> [  232.750645] RAX: 8000000142e002e7 RBX: ffff8881433cae10 RCX: dffffc0000000000
> [  232.750954] RDX: 0000000000000000 RSI: 00007fb47840b000 RDI: 8000000142e002e7
> [  232.751232] RBP: 00007fb47840b000 R08: 0000000000000000 R09: 1ffff110354dff46
> [  232.751514] R10: fffffbfff0cb921d R11: 00000000910da5ce R12: 1ffffffff0c1fcdd
> [  232.751837] R13: 1ffffffff0c23f36 R14: ffff888171628040 R15: 0000000000000000
> [  232.752124] FS:  00007fb4907f86c0(0000) GS:ffff888791f2c000(0000) knlGS:0000000000000000
> [  232.752441] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [  232.752674] CR2: 00007fb47840be00 CR3: 000000015e6dc000 CR4: 00000000000006f0
> [  232.752983] Kernel panic - not syncing: Fatal exception
> [  232.753510] Kernel Offset: disabled
> [  232.754643] ---[ end Kernel panic - not syncing: Fatal exception ]---
> 
> This happens when two concurrent page faults occur within the same PMD range.
> One fault installs a PMD mapping through vmf_insert_pfn_pmd(), while the other
> attempts to install a PTE mapping via vmf_insert_pfn(). The bug is
> triggered because a pmd_trans_huge is not expected when walking the page
> table inside vmf_insert_pfn.
> 
> Avoid this race by adding a huge_fault callback to drm_gem_shmem_vm_ops so that
> PMD-sized mappings are handled through the appropriate huge page fault path.
> 
> Fixes: 211b9a39f261 ("drm/shmem-helper: Map huge pages in fault handler")
> Signed-off-by: Pedro Demarchi Gomes <pedrodemargomes@gmail.com>
> ---
> 
> Changes in v2:
>  - Keep the #ifdef unindented
>  - Create drm_gem_shmem_any_fault to handle faults of any order and use
> drm_gem_shmem_[huge_]fault() as wrappers
> 
> ---
>  drivers/gpu/drm/drm_gem_shmem_helper.c | 67 +++++++++++++++-----------
>  1 file changed, 39 insertions(+), 28 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 7b5a49935ae4..fb8b74ac3057 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -550,36 +550,19 @@ int drm_gem_shmem_dumb_create(struct drm_file *file, struct drm_device *dev,
>  }
>  EXPORT_SYMBOL_GPL(drm_gem_shmem_dumb_create);
>  
> -static bool drm_gem_shmem_try_map_pmd(struct vm_fault *vmf, unsigned long addr,
> -				      struct page *page)
> -{
> -#ifdef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP
> -	unsigned long pfn = page_to_pfn(page);
> -	unsigned long paddr = pfn << PAGE_SHIFT;
> -	bool aligned = (addr & ~PMD_MASK) == (paddr & ~PMD_MASK);
> -
> -	if (aligned &&
> -	    pmd_none(*vmf->pmd) &&
> -	    folio_test_pmd_mappable(page_folio(page))) {
> -		pfn &= PMD_MASK >> PAGE_SHIFT;
> -		if (vmf_insert_pfn_pmd(vmf, pfn, false) == VM_FAULT_NOPAGE)
> -			return true;
> -	}
> -#endif
> -
> -	return false;
> -}
> -
> -static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
> +static vm_fault_t drm_gem_shmem_any_fault(struct vm_fault *vmf,
> +					   unsigned int order)
>  {
>  	struct vm_area_struct *vma = vmf->vma;
>  	struct drm_gem_object *obj = vma->vm_private_data;
>  	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
>  	loff_t num_pages = obj->size >> PAGE_SHIFT;
> -	vm_fault_t ret;
> +	vm_fault_t ret = VM_FAULT_FALLBACK;
>  	struct page **pages = shmem->pages;
>  	pgoff_t page_offset;
>  	unsigned long pfn;
> +	unsigned long paddr;
> +	bool aligned;
>  
>  	/* Offset to faulty address in the VMA. */
>  	page_offset = vmf->pgoff - vma->vm_pgoff;
> @@ -593,13 +576,24 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
>  		goto out;
>  	}
>  
> -	if (drm_gem_shmem_try_map_pmd(vmf, vmf->address, pages[page_offset])) {
> -		ret = VM_FAULT_NOPAGE;
> -		goto out;
> -	}
> -
>  	pfn = page_to_pfn(pages[page_offset]);
> -	ret = vmf_insert_pfn(vma, vmf->address, pfn);
> +	switch (order) {
> +	case 0:
> +		ret = vmf_insert_pfn(vma, vmf->address, pfn);
> +		break;
> +#ifdef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP
> +	case PMD_ORDER:
> +		paddr = pfn << PAGE_SHIFT;
> +		aligned = (vmf->address & ~PMD_MASK) == (paddr & ~PMD_MASK);
> +
> +		if (aligned &&
> +		    folio_test_pmd_mappable(page_folio(pages[page_offset]))) {
> +			pfn &= PMD_MASK >> PAGE_SHIFT;
> +			ret = vmf_insert_pfn_pmd(vmf, pfn, false);
> +		}
> +		break;
> +#endif
> +	}

I'd rather have the pmd vs pte detection done early (before the lock is
taken) so we can return before acquiring the lock and we can get rid of
the order != PMD_ORDER check in drm_gem_shmem_huge_fault(). That, or we
keep the check in drm_gem_shmem_huge_fault() and pass a try_pmd boolean
instead of order to drm_gem_shmem_any_fault().

>  
>   out:
>  	dma_resv_unlock(shmem->base.resv);
> @@ -607,6 +601,20 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
>  	return ret;
>  }
>  
> +static vm_fault_t drm_gem_shmem_huge_fault(struct vm_fault *vmf,
> +					   unsigned int order)
> +{
> +	if (order != PMD_ORDER)
> +		return VM_FAULT_FALLBACK;
> +
> +	return drm_gem_shmem_any_fault(vmf, order);
> +}
> +
> +static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
> +{
> +	return drm_gem_shmem_any_fault(vmf, 0);
> +}
> +
>  static void drm_gem_shmem_vm_open(struct vm_area_struct *vma)
>  {
>  	struct drm_gem_object *obj = vma->vm_private_data;
> @@ -643,6 +651,9 @@ static void drm_gem_shmem_vm_close(struct vm_area_struct *vma)
>  
>  const struct vm_operations_struct drm_gem_shmem_vm_ops = {
>  	.fault = drm_gem_shmem_fault,
> +#ifdef CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP
> +	.huge_fault = drm_gem_shmem_huge_fault,
> +#endif
>  	.open = drm_gem_shmem_vm_open,
>  	.close = drm_gem_shmem_vm_close,
>  };

Claude review: drm/shmem-helper: Fix Map huge page mapping in fault handler

[Claude Code Review Bot]

Overall Series Review

Subject: drm/shmem-helper: Fix Map huge page mapping in fault handler
Author: Pedro Demarchi Gomes <pedrodemargomes@gmail.com>
Patches: 2
Reviewed: 2026-03-14T06:52:13.837152

---

This is a single-patch fix for a real race condition in `drm_gem_shmem_helper.c`. The bug occurs when two concurrent page faults within the same PMD range cause one fault to install a PMD mapping via `vmf_insert_pfn_pmd()` from the `.fault` handler, while another tries to install a PTE mapping via `vmf_insert_pfn()`. The kernel then hits a `BUG()` in `walk_to_pmd()` because it encounters an unexpected `pmd_trans_huge` entry.

The fix correctly separates the PMD mapping path into a `.huge_fault` callback, letting the mm subsystem properly route faults of different orders. The approach follows the same pattern already used by the VFIO PCI drivers (`vfio_pci_core.c`, `nvgrace-gpu/main.c`). The fix is appropriate and the Fixes tag is correct.

There are a couple of minor issues worth noting.

---
Generated by Claude Code Patch Reviewer

Claude review: drm/shmem-helper: Fix Map huge page mapping in fault handler

[Claude Code Review Bot]

Patch Review

**Correctness of the fix:** The approach is sound. By moving the PMD mapping to `.huge_fault`, the mm subsystem handles the dispatch between PTE-level and PMD-level faults, eliminating the race. This is the correct way to do it — the old code was essentially doing huge page mapping opportunistically from the regular `.fault` path, which is inherently racy.

**Unused variable warning when `CONFIG_ARCH_SUPPORTS_PMD_PFNMAP` is not set:**

```c
static vm_fault_t drm_gem_shmem_any_fault(struct vm_fault *vmf,
					   unsigned int order)
{
	...
	unsigned long paddr;
	bool aligned;
```

The variables `paddr` and `aligned` are declared at function scope but only used inside the `#ifdef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP` block in `case PMD_ORDER:`. When that config is disabled, these variables are unused and will produce compiler warnings (`-Wunused-variable`). They should be moved inside the `case PMD_ORDER:` block (with braces around the case body), or `__maybe_unused` could be applied, though moving them is cleaner.

That said, in practice `drm_gem_shmem_any_fault` is only called with `order != 0` from `drm_gem_shmem_huge_fault`, which is only compiled when `CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP` is set, which implies `CONFIG_ARCH_SUPPORTS_PMD_PFNMAP` is also set (per `mm/Kconfig`). So the code is functionally correct, but the warning is still a build issue for configurations without these options enabled (where `.fault` still calls `drm_gem_shmem_any_fault(vmf, 0)` and the variables are dead code).

**Dropped `pmd_none()` check:**

The old code had:
```c
if (aligned &&
    pmd_none(*vmf->pmd) &&
    folio_test_pmd_mappable(page_folio(page))) {
```

The new code drops the `pmd_none(*vmf->pmd)` check:
```c
if (aligned &&
    folio_test_pmd_mappable(page_folio(pages[page_offset]))) {
	pfn &= PMD_MASK >> PAGE_SHIFT;
	ret = vmf_insert_pfn_pmd(vmf, pfn, false);
}
```

This is fine. In the old code, the `pmd_none` check was needed because the code was being called from the regular `.fault` path where the PMD might already be populated with PTEs. In the new `.huge_fault` path, the mm subsystem ensures we're only called when a PMD-level mapping is appropriate, so `vmf_insert_pfn_pmd()` can handle any existing PMD state internally (returning an appropriate error/fallback if needed).

**Minor style nit — commit subject:** "Fix Map huge page mapping" has an odd capitalisation of "Map". Consider: "Fix huge page mapping race in fault handler".

**Missing `default:` case in switch:**

```c
switch (order) {
case 0:
	ret = vmf_insert_pfn(vma, vmf->address, pfn);
	break;
#ifdef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP
case PMD_ORDER:
	...
	break;
#endif
}
```

There's no `default:` case. If an unexpected order is passed, `ret` remains `VM_FAULT_FALLBACK` (from initialization), which is the right behavior, but a `default: break;` would be clearer and would silence any `-Wswitch` warnings if new enum values were ever relevant. The VFIO version (`vfio_pci_vmf_insert_pfn`) explicitly has a `default: return VM_FAULT_FALLBACK;`. This is very minor since `drm_gem_shmem_huge_fault` already filters for `PMD_ORDER` only.

**Overall:** Good fix for a real bug. The pattern matches existing in-tree usage (VFIO). The unused variable issue should be fixed before merging, but otherwise the patch looks correct.

---
Generated by Claude Code Patch Reviewer