On Thu, 12 Mar 2026 12:50:27 -0300
Pedro Demarchi Gomes <[email protected]> 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.
Yikes, looks like we got it wrong between v4 and v6 :-/: v4 had the
.huge_fault implementation, v5 tried moved to .map_pages, and v6 went
back to .fault, but augmented to support huge pages, which I guess is
wrong for the reasons you mentioned.
>
> Fixes: 211b9a39f261 ("drm/shmem-helper: Map huge pages in fault handler")
> Signed-off-by: Pedro Demarchi Gomes <[email protected]>
> ---
> drivers/gpu/drm/drm_gem_shmem_helper.c | 59 +++++++++++++++++++-------
> 1 file changed, 43 insertions(+), 16 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c
> b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 7b5a49935ae4..4cd9080c598b 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -550,24 +550,53 @@ 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)
> +
> +static vm_fault_t drm_gem_shmem_huge_fault(struct vm_fault *vmf,
> + unsigned int order)
> {
> -#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);
> + 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_FALLBACK;
> + struct page **pages = shmem->pages;
> + pgoff_t page_offset;
> + unsigned long pfn;
> + unsigned long paddr;
> + bool aligned;
> + struct page *page;
> +
> + if (order != PMD_ORDER)
> + goto out_unlocked;
> +
> + /* Offset to faulty address in the VMA. */
> + page_offset = vmf->pgoff - vma->vm_pgoff;
> +
> + dma_resv_lock(shmem->base.resv, NULL);
> +
> + if (page_offset >= num_pages ||
> + drm_WARN_ON_ONCE(obj->dev, !shmem->pages) ||
> + shmem->madv < 0) {
> + ret = VM_FAULT_SIGBUS;
> + goto out;
> + }
> +
> + page = pages[page_offset];
> + pfn = page_to_pfn(page);
> + paddr = pfn << PAGE_SHIFT;
> + aligned = (vmf->address & ~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;
> + ret = vmf_insert_pfn_pmd(vmf, pfn, false);
> }
> -#endif
>
> - return false;
> + out:
> + dma_resv_unlock(shmem->base.resv);
> +
> + out_unlocked:
> + return ret;
> }
Do we really need to duplicate everything? Can't we rename
drm_gem_shmem_fault() into drm_gem_shmem_any_fault() with a
`bool try_pmd` passed as an argument, and then have
drm_gem_shmem_[huge_]fault() as wrappers?
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, true);
}
static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
{
return drm_gem_shmem_any_fault(vmf, false);
}
>
> static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
> @@ -593,11 +622,6 @@ 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);
>
> @@ -643,6 +667,9 @@ 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
Let's keep the #ifdef unindented:
#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,
> };
