Re: [PATCH v2] s390: fix HugeTLB vmemmap optimization crash

[David Hildenbrand <david@redhat.com>]

On 28.10.25 22:15, Luiz Capitulino wrote:
> A reproducible crash occurs when enabling HugeTLB vmemmap optimization (HVO)
> on s390. The crash and the proposed fix were worked on an s390 KVM guest
> running on an older hypervisor, as I don't have access to an LPAR. However,
> the same issue should occur on bare-metal.
> 
> Reproducer (it may take a few runs to trigger):
> 
>   # sysctl vm.hugetlb_optimize_vmemmap=1
>   # echo 1 > /proc/sys/vm/nr_hugepages
>   # echo 0 > /proc/sys/vm/nr_hugepages
> 
> Crash log:
> 
> [   52.340369] list_del corruption. prev->next should be 000000d382110008, but was 000000d7116d3880. (prev=000000d7116d3910)
> [   52.340420] ------------[ cut here ]------------
> [   52.340424] kernel BUG at lib/list_debug.c:62!
> [   52.340566] monitor event: 0040 ilc:2 [#1]SMP
> [   52.340573] Modules linked in: ctcm fsm qeth ccwgroup zfcp scsi_transport_fc qdio dasd_fba_mod dasd_eckd_mod dasd_mod xfs ghash_s390 prng des_s390 libdes sha3_512_s390 sha3_256_s390 virtio_net virtio_blk net_failover sha_common failover dm_mirror dm_region_hash dm_log dm_mod paes_s390 crypto_engine pkey_cca pkey_ep11 zcrypt pkey_pckmo pkey aes_s390
> [   52.340606] CPU: 1 UID: 0 PID: 1672 Comm: root-rep2 Kdump: loaded Not tainted 6.18.0-rc3 #1 NONE
> [   52.340610] Hardware name: IBM 3931 LA1 400 (KVM/Linux)
> [   52.340611] Krnl PSW : 0704c00180000000 0000015710cda7fe (__list_del_entry_valid_or_report+0xfe/0x128)
> [   52.340619]            R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:0 PM:0 RI:0 EA:3
> [   52.340622] Krnl GPRS: c0000000ffffefff 0000000100000027 000000000000006d 0000000000000000
> [   52.340623]            000000d7116d35d8 000000d7116d35d0 0000000000000002 000000d7116d39b0
> [   52.340625]            000000d7116d3880 000000d7116d3910 000000d7116d3910 000000d382110008
> [   52.340626]            000003ffac1ccd08 000000d7116d39b0 0000015710cda7fa 000000d7116d37d0
> [   52.340632] Krnl Code: 0000015710cda7ee: c020003e496f	larl	%r2,00000157114a3acc
>             0000015710cda7f4: c0e5ffd5280e	brasl	%r14,000001571077f810
>            #0000015710cda7fa: af000000		mc	0,0
>            >0000015710cda7fe: b9040029		lgr	%r2,%r9
>             0000015710cda802: c0e5ffe5e193	brasl	%r14,0000015710996b28
>             0000015710cda808: e34090080004	lg	%r4,8(%r9)
>             0000015710cda80e: b9040059		lgr	%r5,%r9
>             0000015710cda812: b9040038		lgr	%r3,%r8
> [   52.340643] Call Trace:
> [   52.340645]  [<0000015710cda7fe>] __list_del_entry_valid_or_report+0xfe/0x128
> [   52.340649] ([<0000015710cda7fa>] __list_del_entry_valid_or_report+0xfa/0x128)
> [   52.340652]  [<0000015710a30b2e>] hugetlb_vmemmap_restore_folios+0x96/0x138
> [   52.340655]  [<0000015710a268ac>] update_and_free_pages_bulk+0x64/0x150
> [   52.340659]  [<0000015710a26f8a>] set_max_huge_pages+0x4ca/0x6f0
> [   52.340662]  [<0000015710a273ba>] hugetlb_sysctl_handler_common+0xea/0x120
> [   52.340665]  [<0000015710a27484>] hugetlb_sysctl_handler+0x44/0x50
> [   52.340667]  [<0000015710b53ffa>] proc_sys_call_handler+0x17a/0x280
> [   52.340672]  [<0000015710a90968>] vfs_write+0x2c8/0x3a0
> [   52.340676]  [<0000015710a90bd2>] ksys_write+0x72/0x100
> [   52.340679]  [<00000157111483a8>] __do_syscall+0x150/0x318
> [   52.340682]  [<0000015711153a5e>] system_call+0x6e/0x90
> [   52.340684] Last Breaking-Event-Address:
> [   52.340684]  [<000001571077f85c>] _printk+0x4c/0x58
> [   52.340690] Kernel panic - not syncing: Fatal exception: panic_on_oops
> 
> This issue was introduced by commit f13b83fdd996 ("hugetlb: batch TLB
> flushes when freeing vmemmap"). Before that change, the HVO
> implementation called flush_tlb_kernel_range() each time a vmemmap
> PMD split and remapping was performed. The mentioned commit changed this
> to issue a few flush_tlb_all() calls after performing all remappings.
> 
> However, on s390, flush_tlb_kernel_range() expands to
> __tlb_flush_kernel() while flush_tlb_all() is not implemented. As a
> result, we went from flushing the TLB for every remapping to no flushing
> at all.
> 
> This commit fixes this by implementing flush_tlb_all() on s390 as an
> alias to __tlb_flush_global(). This should cause a flush on all TLB
> entries on all CPUs as expected by the flush_tlb_all() semantics.
> 
> Fixes: f13b83fdd996 ("hugetlb: batch TLB flushes when freeing vmemmap")
> Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
> ---

Nice finding!

Makes me wonder whether the default flush_tlb_all() should actually map 
to a BUILD_BUG(), such that we don't silently not-flush on archs that 
don't implement it.

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Cheers

David / dhildenb