Re: [PATCH] btrfs: harden __reserve_bytes() with space_info==NULL

[Qu Wenruo <wqu@suse.com>]

在 2025/12/14 08:13, Kai Krakow 写道:
> Am Sa., 13. Dez. 2025 um 22:15 Uhr schrieb Qu Wenruo <wqu@suse.com>:
[...]
>>
>> So if you really want to change a member, do a proper in-memory update
>> only, and find a way (e.g. a new dirty dev list) to tell the fs to
>> update the device item at commit time.
> 
> I don't want to steal your time,

I'm totally fine discussing the implementation details here.

> but is this a better approach?
> https://gist.github.com/kakra/aa3eb8473cc05c1d3dd000160a5ee481

Unfortunately as long as if you're trying to do any metadata 
modification, e.g. calling btrfs_update_device(), it will be a huge change.


My idea would be something like this:

btrfs_dev_info_type_store()
{
	btrfs_device *device = container_of();

	/* Do the proper locking. */

	WRITE_ONCE(device->type, type);
	if (!list_empty(&dev->dirty_list))
		list_add_tail(&fs_info->dirty_dev_list, &dev->dirty_list);

	return len;
}


Then inside btrfs_commit_transaction(), I do not yet have a good idea on 
the timing, but I guess it can done before btrfs_start_delalloc_flush().

Do something like this to write those dirty devices to chunk tree:

btrfs_commit_transaction()
{
	list_for_each_entry(dev, &fs_info->dirty_dev_list, dirty_list) {
		ret = btrfs_update_device(dev);
	}

	/* The remaining code. */
	ret = btrfs_start_delalloc_flush();
}


> 
> Thanks,
> Kai
> 
>> It's much safer and avoid a new and untested direct way to trigger
>> metadata modification.
>>
>> Thanks,
>> Qu
>>
>>>
>>> So I should be safe there even without this patch.
>>>
>>> Thanks,
>>> Kai
>>>
>>>>>> Noticed an oops with these patches when doing echo 1 >devinfo/2/type
>>>>>> while mount is still ongoing. My btrfs is big so the mount takes
>>>>>> 20-30 minutes. Reboot and wait until mount is complete and this
>>>>>> worked fine.
>>>>>
>>>>> BUG: kernel NULL pointer dereference, address: 0000000000000008
>>>>> PGD 0 P4D 0
>>>>> Oops: Oops: 0002 [#1] PREEMPT SMP NOPTI
>>>>> CPU: 4 UID: 0 PID: 3520 Comm: bash Not tainted 6.12.52-dirty #2
>>>>> Hardware name: Penguin Computing Relion 1900/MD90-FS0-ZB-XX, BIOS R15 06/25/2018
>>>>> RIP: 0010:_raw_spin_lock+0x17/0x30
>>>>> Code: 44 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 0f 1f 44 00 00 65 ff 05 e8 c0 d8 5e 31 c0 ba 01 00 00 00 0f b1 17 75 05 c3 cc cc cc cc 89 c6 e9 97 01 00 00 0f 1f 80 00
>>>>> RSP: 0018:ffffbc13a95837c8 EFLAGS: 00010246
>>>>> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
>>>>> RDX: 0000000000000001 RSI: 0000000000000000 RDI: 0000000000000008
>>>>> RBP: 0000000000000008 R08: ffffbc13a9583a07 R09: 0000000000000001
>>>>> R10: d800000000000000 R11: 0000000000000001 R12: ffff9bee913db000
>>>>> R13: 0000000000000000 R14: 00000000fffffffb R15: ffff9bee913db000
>>>>> FS: 00007fd6e270f740(0000) GS:ffff9bfddfc00000(0000) knlGS:0000000000000000
>>>>> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>>> CR2: 0000000000000008 CR3: 00000008d9986004 CR4: 00000000003706f0
>>>>> Call Trace:
>>>>>
>>>>> __reserve_bytes+0x70/0x720 [btrfs]
>>>>> ? get_page_from_freelist+0x343/0x1570
>>>>> btrfs_reserve_metadata_bytes+0x1d/0xd0 [btrfs]
>>>>> btrfs_use_block_rsv+0x153/0x220 [btrfs]
>>>>> btrfs_alloc_tree_block+0x83/0x580 [btrfs]
>>>>> btrfs_force_cow_block+0x129/0x620 [btrfs]
>>>>> btrfs_cow_block+0xcd/0x230 [btrfs]
>>>>> btrfs_search_slot+0x566/0xd60 [btrfs]
>>>>> ? kmem_cache_alloc_noprof+0x106/0x2f0
>>>>> btrfs_update_device+0x91/0x1d0 [btrfs]
>>>>> btrfs_devinfo_type_store+0xb8/0x140 [btrfs]
>>>>> kernfs_fop_write_iter+0x14c/0x200
>>>>> vfs_write+0x289/0x440
>>>>> ksys_write+0x6d/0xf0
>>>>> trace_clock_x86_tsc+0x20/0x20
>>>>> ? do_wp_page+0x838/0xf90
>>>>> ? __do_sys_newfstat+0x68/0x70
>>>>> ? __pte_offset_map+0x1b/0xf0
>>>>> ? __handle_mm_fault+0xa6c/0x10f0
>>>>> ? __count_memcg_events+0x53/0xf0
>>>>> ? handle_mm_fault+0x1c4/0x2d0
>>>>> ? do_user_addr_fault+0x334/0x620
>>>>> ? arch_exit_to_user_mode_prepare.isra.0+0x11/0x90
>>>>> entry_SYSCALL_64_after_hwframe+0x76/0x7e
>>>>> RIP: 0033:0x7fd6e27a1687
>>>>> Code: 48 89 fa 4c 89 df e8 58 b3 00 00 8b 93 08 03 00 00 59 5e 48 83 f8 fc 74 1a 5b c3 0f 1f 84 00 00 00 00 00 48 8b 44 24 10 0f 05 <5b> c3 0f 1f 80 00 00 00 00 83 e2 39 83 fa 08 75 de e8 23 ff ff ff
>>>>> RSP: 002b:00007ffecb401260 EFLAGS: 00000202 ORIG_RAX: 0000000000000001
>>>>> RAX: ffffffffffffffda RBX: 00007fd6e270f740 RCX: 00007fd6e27a1687
>>>>> RDX: 0000000000000002 RSI: 0000557a2c38ad20 RDI: 0000000000000001
>>>>> RBP: 0000557a2c38ad20 R08: 0000000000000000 R09: 0000000000000000
>>>>> R10: 0000000000000000 R11: 0000000000000202 R12: 0000000000000002
>>>>> R13: 00007fd6e28fa5c0 R14: 00007fd6e28f7e80 R15: 0000000000000000
>>>>>
>>>>> Modules linked in: rpcsec_gss_krb5 nfsv3 nfsv4 dns_resolver nfs netfs zram lz4hc_compress lz4_compress dm_crypt bonding tls ipmi_ssif intel_rapl_msr nfsd binfmt_misc auth_rpcgss nfs_acl lockd grace intel_rapl_common intel_uncore_frequency intel_uncore_frequency_common sb_edac x86_pkg_temp_thermal intel_powerclamp coretemp rapl intel_cstate s>
>>>>> intel_pmc_bxt ixgbe ehci_pci iTCO_vendor_support xfrm_algo gf128mul libata mpt3sas xhci_hcd ehci_hcd watchdog crypto_simd mdio_devres libphy cryptd raid_class usbcore scsi_transport_sas mdio igb scsi_mod wmi usb_common i2c_i801 lpc_ich scsi_common i2c_smbus i2c_algo_bit dca
>>>>> CR2: 0000000000000008
>>>>> ---[ end trace 0000000000000000 ]---
>>>>> RIP: 0010:_raw_spin_lock+0x17/0x30
>>>>> Code: 44 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 0f 1f 44 00 00 65 ff 05 e8 c0 d8 5e 31 c0 ba 01 00 00 00 0f b1 17 75 05 c3 cc cc cc cc 89 c6 e9 97 01 00 00 0f 1f 80 00
>>>>> RSP: 0018:ffffbc13a95837c8 EFLAGS: 00010246
>>>>> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
>>>>> RDX: 0000000000000001 RSI: 0000000000000000 RDI: 0000000000000008
>>>>> RBP: 0000000000000008 R08: ffffbc13a9583a07 R09: 0000000000000001
>>>>> R10: d800000000000000 R11: 0000000000000001 R12: ffff9bee913db000
>>>>> R13: 0000000000000000 R14: 00000000fffffffb R15: ffff9bee913db000
>>>>> FS: 00007fd6e270f740(0000) GS:ffff9bfddfc00000(0000) knlGS:0000000000000000
>>>>> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>>> CR2: 0000000000000008 CR3: 00000008d9986004 CR4: 00000000003706f0
>>>>>
>>>>> Reported-by: Eli Venter <eli@genedx.com>
>>>>> Signed-off-by: Kai Krakow <kai@kaishome.de>
>>>>> ---
>>>>>     fs/btrfs/space-info.c | 8 ++++++++
>>>>>     1 file changed, 8 insertions(+)
>>>>>
>>>>> diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
>>>>> index 97452fb5d29b..cbb6c4924850 100644
>>>>> --- a/fs/btrfs/space-info.c
>>>>> +++ b/fs/btrfs/space-info.c
>>>>> @@ -1752,6 +1752,14 @@ static int __reserve_bytes(struct btrfs_fs_info *fs_info,
>>>>>                 ASSERT(flush != BTRFS_RESERVE_FLUSH_EVICT);
>>>>>         }
>>>>>
>>>>> +     /*
>>>>> +      * During mount, the global block reserve might not have its space_info
>>>>> +      * initialized yet. If we try to reserve bytes in this state (e.g. via
>>>>> +      * early sysfs writes), we must not crash.
>>>>> +      */
>>>>> +     if (unlikely(!space_info))
>>>>> +             return -EBUSY;
>>>>> +
>>>>>         if (flush == BTRFS_RESERVE_FLUSH_DATA)
>>>>>                 async_work = &fs_info->async_data_reclaim_work;
>>>>>         else
>>>>
>>