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 NTnCgIC3 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Oct 14, 2025 at 05:31:24PM +0800, Hao Ge wrote: > From: Hao Ge > > We should not reuse the first bit for OBJEXTS_ALLOC_FAIL. > This is because the following scenarios may be encountered: > > Under heavy system load, certain sequences of events can trigger the Hi Hao, thanks for catching it! It's late at night and my brain is tired so I may be missing something, but let me leave comment anyway... > VM_BUG_ON_FOLIO(folio->memcg_data & MEMCG_DATA_OBJEXTS, folio) check: Should we check (folio->memcg_data != OBJEXTS_ALLOC_FAIL) && (folio->memcg_data & MEMCG_DATA_OBJEXTS) instead then? Not clearing a valid folio->memcg_data is considered an error, but freeing a folio that is marked OBJEXTS_ALLOC_FAIL isn't. > 1. High system pressure may cause objext allocation failure for a slab. > 2. When objext allocation fails, slab->obj_exts is set to > OBJEXTS_ALLOC_FAIL (value 1). > 3. Later, this slab may enter the release process. > 4. During release of the associated folio, the existing > VM_BUG_ON_FOLIO check validates folio->memcg_data. > If the MEMCG_DATA_OBJEXTS bit is unexpectedly > set here, the bug check gets triggered. > > We have obtained the following logs: > [ 7108.343437] page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffff0002deb97600 pfn:0x31eb96 > [ 7108.343482] head: order:1 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0 > [ 7108.343500] memcg:1 > [ 7108.343507] flags: 0x17ffff800000040(head|node=0|zone=2|lastcpupid=0xfffff) > [ 7108.343523] raw: 017ffff800000040 ffff0000c000cac0 dead000000000100 0000000000000000 > [ 7108.343528] raw: ffff0002deb97600 0000000000240000 00000000ffffffff 0000000000000001 > [ 7108.343534] head: 017ffff800000040 ffff0000c000cac0 dead000000000100 0000000000000000 > [ 7108.343539] head: ffff0002deb97600 0000000000240000 00000000ffffffff 0000000000000001 > [ 7108.343562] head: 017ffff800000001 fffffdffcb7ae581 00000000ffffffff 00000000ffffffff > [ 7108.343569] head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000002 > [ 7108.343574] page dumped because: VM_BUG_ON_FOLIO(folio->memcg_data & MEMCG_DATA_OBJEXTS) > [ 7108.343601] ------------[ cut here ]------------ > [ 7108.343607] kernel BUG at ./include/linux/memcontrol.h:537! > [ 7108.343617] Internal error: Oops - BUG: 00000000f2000800 [#1] SMP > [ 7108.345751] Modules linked in: squashfs isofs vhost_vsock vhost_net vmw_vsock_virtio_transport_common vfio_iommu_type1 vhost vfio vsock vhost_iotlb iommufd tap binfmt_misc nfsv3 nfs_acl nfs lockd grace netfs tls rds dns_resolver tun brd overlay ntfs3 exfat btrfs blake2b_generic xor xor_neon raid6_pq loop sctp ip6_udp_tunnel udp_tunnel 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 rfkill ip_set sunrpc vfat fat joydev sg sch_fq_codel nfnetlink virtio_gpu drm_client_lib virtio_dma_buf drm_shmem_helper sr_mod drm_kms_helper cdrom drm ghash_ce virtio_net virtio_scsi backlight virtio_console virtio_blk net_failover failover virtio_mmio dm_mirror dm_region_hash dm_log dm_multipath dm_mod fuse i2c_dev virtio_pci virtio_pci_legacy_dev virtio_pci_modern_dev virtio virtio_ring autofs4 aes_neon_bs aes_ce_blk [last unloaded: hwpoison_inject] > [ 7108.355662] CPU: 7 UID: 0 PID: 4470 Comm: kylin-process-m Kdump: loaded Not tainted 6.18.0-rc1-dirty #54 PREEMPT(voluntary) > [ 7108.356864] Hardware name: QEMU KVM Virtual Machine, BIOS unknown 2/2/2022 > [ 7108.357621] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--) > [ 7108.358981] pc : __free_frozen_pages+0xf18/0x18e8 > [ 7108.359834] lr : __free_frozen_pages+0xf18/0x18e8 > [ 7108.360379] sp : ffff8000a2bb7580 > [ 7108.360786] x29: ffff8000a2bb7580 x28: fffffdffcb7ae580 x27: fffffdffcb7ae580 > [ 7108.362013] x26: fffffdffcb7ae588 x25: 1fffffbff96f5cb1 x24: 1fffffbff96f5cb0 > [ 7108.362804] x23: ffff8000839d6ba0 x22: ffff8000839d6000 x21: 0000000000000000 > [ 7108.363596] x20: 0000000000000000 x19: 0000000000000001 x18: 0000000000000000 > [ 7108.364393] x17: 445f47434d454d20 x16: 2620617461645f67 x15: 636d656d3e2d6f69 > [ 7108.365498] x14: 6c6f66284f494c4f x13: 0000000000000001 x12: ffff600063fece93 > [ 7108.366317] x11: 1fffe00063fece92 x10: ffff600063fece92 x9 : dfff800000000000 > [ 7108.367610] x8 : 00009fff9c01316e x7 : ffff00031ff67493 x6 : 0000000000000001 > [ 7108.368455] x5 : ffff00031ff67490 x4 : ffff600063fece93 x3 : 0000000000000000 > [ 7108.369276] x2 : 0000000000000000 x1 : ffff000103fe5d40 x0 : 000000000000004c > [ 7108.370140] Call trace: > [ 7108.370463] __free_frozen_pages+0xf18/0x18e8 (P) > [ 7108.371011] free_frozen_pages+0x1c/0x30 > [ 7108.372040] __free_slab+0xd0/0x250 > [ 7108.372471] free_slab+0x38/0x118 > [ 7108.372882] free_to_partial_list+0x1d4/0x340 > [ 7108.373813] __slab_free+0x24c/0x348 > [ 7108.374253] ___cache_free+0xf0/0x110 > [ 7108.374699] qlist_free_all+0x78/0x130 > [ 7108.375156] kasan_quarantine_reduce+0x114/0x148 > [ 7108.375695] __kasan_slab_alloc+0x7c/0xb0 > [ 7108.376668] kmem_cache_alloc_noprof+0x164/0x5c8 > [ 7108.377206] __alloc_object+0x44/0x1f8 > [ 7108.377659] __create_object+0x34/0xc8 > [ 7108.378196] kmemleak_alloc+0xb8/0xd8 > [ 7108.378644] kmem_cache_alloc_noprof+0x368/0x5c8 > [ 7108.379224] getname_flags.part.0+0xa4/0x610 > [ 7108.379733] getname_flags+0x80/0xd8 > [ 7108.380169] do_sys_openat2+0xb4/0x178 > [ 7108.380921] __arm64_sys_openat+0x134/0x1d0 > [ 7108.381952] invoke_syscall+0xd4/0x258 > [ 7108.382408] el0_svc_common.constprop.0+0xb4/0x240 > [ 7108.382965] do_el0_svc+0x48/0x68 > [ 7108.383375] el0_svc+0x40/0xe0 > [ 7108.383757] el0t_64_sync_handler+0xa0/0xe8 > [ 7108.384465] el0t_64_sync+0x1ac/0x1b0 > [ 7108.385284] Code: 91398021 aa1b03e0 91138021 97fd35e3 (d4210000) > [ 7108.386553] SMP: stopping secondary CPUs > [ 7108.389714] Starting crashdump kernel... > [ 7108.390190] Bye! > > So, introduce __SECOND_OBJEXT_FLAG for objext_flags, adjust > the corresponding order accordingly, and ensure that OBJEXTS_ALLOC_FAIL > is no longer reused. > > Fixes: 7612833192d5 ("slab: Reuse first bit for OBJEXTS_ALLOC_FAIL") Hmm using a new bit was suggested at that time, but that would require bumping up the alignment when allocating slabobj_ext array? (see alloc_slab_obj_exts()) And we can still distinguish two cases where 1) MEMCG_DATA_OBJEXTS is set, but upper bits are not set, so it should mean obj_exts allocation failed (OBJEXTS_ALLOC_FAIL), thus do not report error, or 2) MEMCG_DATA_OBJEXTS is set, and upper bits are also set, so someone did not clear a valid folio->memcg_data before freeing the folio (report error). without introducing a new bit, right? > Signed-off-by: Hao Ge > --- > include/linux/memcontrol.h | 16 ++++++---------- > 1 file changed, 6 insertions(+), 10 deletions(-) > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h > index 873e510d6f8d..8ea023944fac 100644 > --- a/include/linux/memcontrol.h > +++ b/include/linux/memcontrol.h > @@ -341,27 +341,23 @@ enum page_memcg_data_flags { > __NR_MEMCG_DATA_FLAGS = (1UL << 2), > }; > > -#define __OBJEXTS_ALLOC_FAIL MEMCG_DATA_OBJEXTS > #define __FIRST_OBJEXT_FLAG __NR_MEMCG_DATA_FLAGS > +#define __SECOND_OBJEXT_FLAG (__FIRST_OBJEXT_FLAG << 1) > > #else /* CONFIG_MEMCG */ > > -#define __OBJEXTS_ALLOC_FAIL (1UL << 0) > #define __FIRST_OBJEXT_FLAG (1UL << 0) > +#define __SECOND_OBJEXT_FLAG (1UL << 0) > > #endif /* CONFIG_MEMCG */ > > enum objext_flags { > - /* > - * Use bit 0 with zero other bits to signal that slabobj_ext vector > - * failed to allocate. The same bit 0 with valid upper bits means > - * MEMCG_DATA_OBJEXTS. > - */ > - OBJEXTS_ALLOC_FAIL = __OBJEXTS_ALLOC_FAIL, > + /* slabobj_ext vector failed to allocate */ > + OBJEXTS_ALLOC_FAIL = __FIRST_OBJEXT_FLAG, > /* slabobj_ext vector allocated with kmalloc_nolock() */ > - OBJEXTS_NOSPIN_ALLOC = __FIRST_OBJEXT_FLAG, > + OBJEXTS_NOSPIN_ALLOC = __SECOND_OBJEXT_FLAG, > /* the next bit after the last actual flag */ > - __NR_OBJEXTS_FLAGS = (__FIRST_OBJEXT_FLAG << 1), > + __NR_OBJEXTS_FLAGS = (__SECOND_OBJEXT_FLAG << 1), > }; > > #define OBJEXTS_FLAGS_MASK (__NR_OBJEXTS_FLAGS - 1) > -- > 2.25.1 > -- Cheers, Harry / Hyeonggon