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b=nKH3LFNo05J4xcOB7wo7u4PTK9A3GgUiPnzFayahwAlQCkAceKiuD/c/hRetT0nWh mNcW2sJzpV+GVvW5guxgG3ZRht18SFINqfvw+mFVCZRH4oDNvG4BP7IxUP3ucustbw oRamMD5BprKaRg37CVMfPtgHx3oYMYdq02v2C+Mw67YJhD8QsoGNNWVIuWk5KVWnT5 oZYAd3W6rGvRxuP2wkex2t81DbvEz1iwCGWpjZ/ubr9KbJfoD3zLLv8HsRg1Pqw1Cz Q1u+uIghqDM0e+vMTPGTsjZPWGzh4YNPOUeiPIg3GPTJKiYxe3VYzpTfBNHEckCn32 oGeYrL+zwAxsA== Received: from fedora (unknown [IPv6:2a01:e0a:2c:6930:d919:a6e:5ea1:8a9f]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (prime256v1) server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) (Authenticated sender: bbrezillon) by bali.collaboradmins.com (Postfix) with ESMTPSA id 2446D17E0D98; Thu, 12 Mar 2026 17:32:32 +0100 (CET) Date: Thu, 12 Mar 2026 17:32:27 +0100 From: Boris Brezillon To: Pedro Demarchi Gomes Cc: Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , Loic Molinari , dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] drm/shmem-helper: Fix Map huge page mapping in fault handler Message-ID: <20260312173227.7e9f0389@fedora> In-Reply-To: <20260312155027.1682606-1-pedrodemargomes@gmail.com> References: <20260312155027.1682606-1-pedrodemargomes@gmail.com> Organization: Collabora X-Mailer: Claws Mail 4.3.1 (GTK 3.24.51; x86_64-redhat-linux-gnu) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Thu, 12 Mar 2026 12:50:27 -0300 Pedro Demarchi Gomes 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] > [ 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] > [ 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 > --- > 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, > };