From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-178.mta0.migadu.com (out-178.mta0.migadu.com [91.218.175.178]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 119381DFCB for ; Tue, 14 Oct 2025 11:18:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.178 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760440741; cv=none; b=phRieFsKtW5hcGWxUYBTjCB7QMkfMwZIUTJMip01gbsLDT1BhCONCN9WXC7wWkqVq3R0wAg7G/9+1ZBTLXH+MHX4eaQtiFhoP93qfj1hMaoWeLiup0EvB1Ad/s/o8PF9YiBzdIRyxcayj4RliovYeZii1veI2JTaYSNtwNI//hs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760440741; c=relaxed/simple; bh=8Vf9bnpqdQVY+FOVOnz/MQpT4U8GRNrk27/JWpR2vNc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=L64keBMEXnsfDVTsp43Z/cZk3/k1q7FAXdCasHX0YPojs21axmm7c8udiO3wpBuwzhL6hXnf7yHYE2sS5cSn+oyyF761REh4QGsPpr3kgMegZJBnI3lPuIkPzX19gFoPjFoJRX3vgvs4fKJIy2+wguUg+KkoRD4SDL7qUSBLGLk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=b7iY4Mnk; arc=none smtp.client-ip=91.218.175.178 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="b7iY4Mnk" Message-ID: <0fb23d43-95cb-4a1a-b369-e461ad38ad3e@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1760440737; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ez4oHLXsZBt/HMrDfDptsdIpJrVGoNbHGfMaBzQh63k=; b=b7iY4Mnk5SuXmI/+8Y6nBdh+JbrNanjkpbHHt50QTg26ZmE3iQMSZvep6pbUvJbsb3zFHZ 3ybWtkDUYM9zclzxox9LvOxvCOXRDnCBbRr6GgSwCeWf5pOBKIaRhIU6rVCi6N68OSqJux AH5YHJ4xngbSf0Jy+WJ7QQmHPIk9NRs= Date: Tue, 14 Oct 2025 19:18:03 +0800 Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH] slab: Introduce __SECOND_OBJEXT_FLAG for objext_flags To: Harry Yoo Cc: Vlastimil Babka , Alexei Starovoitov , Andrew Morton , Johannes Weiner , Shakeel Butt , Michal Hocko , Roman Gushchin , Muchun Song , Suren Baghdasaryan , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hao Ge References: <20251014093124.300012-1-hao.ge@linux.dev> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Hao Ge In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 2025/10/14 18:27, Harry Yoo wrote: > 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. Hi Harry Thank you very much for taking the time to review my patch amid your busy work. It was also that I didn’t express myself clearly in the following paragraph. >> 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). The sentence "2. When objext allocation fails, slab->obj_exts is set to OBJEXTS_ALLOC_FAIL (value 1)." should be converted to this one: "2. When objext allocation fails, slab->obj_exts is set to OBJEXTS_ALLOC_FAIL, and OBJEXTS_ALLOC_FAIL is actually equivalent to MEMCG_DATA_OBJEXTS." So the root cause of this issue lies here as well—because OBJEXTS_ALLOC_FAIL and MEMCG_DATA_OBJEXTS are reusing the same bit. Thanks Best Regards Hao >> 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 >>