Re: [PATCH v7 0/6] Per-VMA lock support for swap and userfaults

[David Hildenbrand <david@redhat.com>]

On 30.06.23 23:19, Suren Baghdasaryan wrote:
> When per-VMA locks were introduced in [1] several types of page faults
> would still fall back to mmap_lock to keep the patchset simple. Among them
> are swap and userfault pages. The main reason for skipping those cases was
> the fact that mmap_lock could be dropped while handling these faults and
> that required additional logic to be implemented.
> Implement the mechanism to allow per-VMA locks to be dropped for these
> cases.
> First, change handle_mm_fault to drop per-VMA locks when returning
> VM_FAULT_RETRY or VM_FAULT_COMPLETED to be consistent with the way
> mmap_lock is handled. Then change folio_lock_or_retry to accept vm_fault
> and return vm_fault_t which simplifies later patches. Finally allow swap
> and uffd page faults to be handled under per-VMA locks by dropping per-VMA
> and retrying, the same way it's done under mmap_lock.
> Naturally, once VMA lock is dropped that VMA should be assumed unstable
> and can't be used.
> 
> Changes since v6 posted at [2]
> - 4/6 replaced the ternary operation in folio_lock_or_retry,
> per Matthew Wilcox
> - 4/6 changed return code description for __folio_lock_or_retry
> per Matthew Wilcox
> 
> Note: patch 3/6 will cause a trivial merge conflict in arch/arm64/mm/fault.c
> when applied over mm-unstable branch due to a patch from ARM64 tree [3]
> which is missing in mm-unstable.
> 
> [1] https://lore.kernel.org/all/20230227173632.3292573-1-surenb@google.com/
> [2] https://lore.kernel.org/all/20230630020436.1066016-1-surenb@google.com/
> [3] https://lore.kernel.org/all/20230524131305.2808-1-jszhang@kernel.org/
> 
> Suren Baghdasaryan (6):
>    swap: remove remnants of polling from read_swap_cache_async
>    mm: add missing VM_FAULT_RESULT_TRACE name for VM_FAULT_COMPLETED
>    mm: drop per-VMA lock when returning VM_FAULT_RETRY or
>      VM_FAULT_COMPLETED
>    mm: change folio_lock_or_retry to use vm_fault directly
>    mm: handle swap page faults under per-VMA lock
>    mm: handle userfaults under VMA lock

On mm/mm-unstable I get running the selftests:

Testing sigbus-wp on shmem... [  383.215804] mm ffff9666078e5280 task_size 140737488351232
[  383.215804] get_unmapped_area ffffffffad03b980
[  383.215804] mmap_base 140378441285632 mmap_legacy_base 47254353883136
[  383.215804] pgd ffff966608960000 mm_users 1 mm_count 6 pgtables_bytes 126976 map_count 28
[  383.215804] hiwater_rss 6183 hiwater_vm 8aa7 total_vm 8aa7 locked_vm 0
[  383.215804] pinned_vm 0 data_vm 844 exec_vm 1a4 stack_vm 21
[  383.215804] start_code 402000 end_code 408f09 start_data 40ce10 end_data 40d500
[  383.215804] start_brk 17fe000 brk 1830000 start_stack 7ffecbbe08e0
[  383.215804] arg_start 7ffecbbe1c6f arg_end 7ffecbbe1c81 env_start 7ffecbbe1c81 env_end 7ffecbbe1fe6
[  383.215804] binfmt ffffffffaf3efe40 flags 80000cd
[  383.215804] ioctx_table 0000000000000000
[  383.215804] owner ffff96660d4a4000 exe_file ffff966285501a00
[  383.215804] notifier_subscriptions 0000000000000000
[  383.215804] numa_next_scan 4295050919 numa_scan_offset 0 numa_scan_seq 0
[  383.215804] tlb_flush_pending 0
[  383.215804] def_flags: 0x0()
[  383.236255] ------------[ cut here ]------------
[  383.237537] kernel BUG at include/linux/mmap_lock.h:66!
[  383.238897] invalid opcode: 0000 [#1] PREEMPT SMP PTI
[  383.240114] CPU: 37 PID: 1482 Comm: uffd-unit-tests Not tainted 6.5.0-rc4+ #68
[  383.242513] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-1.fc38 04/01/2014
[  383.244936] RIP: 0010:find_vma+0x3a/0x40
[  383.246200] Code: 48 89 34 24 48 85 c0 74 1c 48 83 c7 40 48 c7 c2 ff ff ff ff 48 89 e6 e8 a4 29 ba 00
[  383.251084] RSP: 0000:ffffae3745b6beb0 EFLAGS: 00010282
[  383.252781] RAX: 0000000000000314 RBX: ffff9666078e5280 RCX: 0000000000000000
[  383.255073] RDX: 0000000000000001 RSI: ffffffffae8f69c3 RDI: 00000000ffffffff
[  383.257352] RBP: 0000000000000000 R08: 0000000000000000 R09: ffffae3745b6bc48
[  383.259369] R10: 0000000000000003 R11: ffff9669fff46fe8 R12: 0000000044401028
[  383.261570] R13: ffff9666078e5338 R14: ffffae3745b6bf58 R15: 0000000000000400
[  383.263499] FS:  00007fac671c5740(0000) GS:ffff9669efbc0000(0000) knlGS:0000000000000000
[  383.265483] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  383.266847] CR2: 0000000044401028 CR3: 0000000488960006 CR4: 0000000000770ee0
[  383.268532] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  383.270206] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  383.271905] PKRU: 55555554
[  383.272593] Call Trace:
[  383.273215]  <TASK>
[  383.273774]  ? die+0x32/0x80
[  383.274510]  ? do_trap+0xd6/0x100
[  383.275326]  ? find_vma+0x3a/0x40
[  383.276152]  ? do_error_trap+0x6a/0x90
[  383.277072]  ? find_vma+0x3a/0x40
[  383.277899]  ? exc_invalid_op+0x4c/0x60
[  383.278846]  ? find_vma+0x3a/0x40
[  383.279675]  ? asm_exc_invalid_op+0x16/0x20
[  383.280698]  ? find_vma+0x3a/0x40
[  383.281527]  lock_mm_and_find_vma+0x3f/0x270
[  383.282570]  do_user_addr_fault+0x1e4/0x660
[  383.283591]  exc_page_fault+0x73/0x170
[  383.284509]  asm_exc_page_fault+0x22/0x30
[  383.285486] RIP: 0033:0x404428
[  383.286265] Code: 48 89 85 18 ff ff ff e9 dc 00 00 00 48 8b 15 9f 92 00 00 48 8b 05 80 92 00 00 48 03
[  383.290566] RSP: 002b:00007ffecbbe05c0 EFLAGS: 00010206
[  383.291814] RAX: 0000000044401028 RBX: 00007ffecbbe08e8 RCX: 00007fac66e93c18
[  383.293502] RDX: 0000000044400000 RSI: 0000000000000001 RDI: 0000000000000000
[  383.295175] RBP: 00007ffecbbe06c0 R08: 00007ffecbbe05c0 R09: 00007ffecbbe06c0
[  383.296857] R10: 0000000000000008 R11: 0000000000000246 R12: 0000000000000000
[  383.298533] R13: 00007ffecbbe08f8 R14: 000000000040ce18 R15: 00007fac67206000
[  383.300203]  </TASK>
[  383.300775] Modules linked in: rfkill intel_rapl_msr intel_rapl_common intel_uncore_frequency_commong
[  383.309661] ---[ end trace 0000000000000000 ]---
[  383.310795] RIP: 0010:find_vma+0x3a/0x40
[  383.311771] Code: 48 89 34 24 48 85 c0 74 1c 48 83 c7 40 48 c7 c2 ff ff ff ff 48 89 e6 e8 a4 29 ba 00
[  383.316081] RSP: 0000:ffffae3745b6beb0 EFLAGS: 00010282
[  383.317346] RAX: 0000000000000314 RBX: ffff9666078e5280 RCX: 0000000000000000
[  383.319050] RDX: 0000000000000001 RSI: ffffffffae8f69c3 RDI: 00000000ffffffff
[  383.320767] RBP: 0000000000000000 R08: 0000000000000000 R09: ffffae3745b6bc48
[  383.322468] R10: 0000000000000003 R11: ffff9669fff46fe8 R12: 0000000044401028
[  383.324164] R13: ffff9666078e5338 R14: ffffae3745b6bf58 R15: 0000000000000400
[  383.325870] FS:  00007fac671c5740(0000) GS:ffff9669efbc0000(0000) knlGS:0000000000000000
[  383.327795] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  383.329177] CR2: 0000000044401028 CR3: 0000000488960006 CR4: 0000000000770ee0
[  383.330885] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  383.332592] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  383.334287] PKRU: 55555554


Which ends up being

VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_lock), mm);

I did not check if this is also the case on mainline, and if this series is responsible.

-- 
Cheers,

David / dhildenb