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BUG: KCSAN: data-race in ext4_nonda_switch / percpu_counter_add_batch

read to 0xffff88821878b158 of 8 bytes by task 10472 on cpu 0:
 ext4_nonda_switch+0x5a/0x140 fs/ext4/inode.c:2990
 ext4_page_mkwrite+0x748/0xce0 fs/ext4/inode.c:6215
 do_page_mkwrite+0x100/0x210 mm/memory.c:2208
 do_shared_fault mm/memory.c:3554 [inline]
 do_fault mm/memory.c:3622 [inline]
 handle_pte_fault mm/memory.c:3849 [inline]
 __handle_mm_fault+0x2089/0x2c70 mm/memory.c:3973
 handle_mm_fault+0x21b/0x530 mm/memory.c:4010
 do_user_addr_fault arch/x86/mm/fault.c:1441 [inline]
 __do_page_fault+0x3fb/0x9e0 arch/x86/mm/fault.c:1506
 do_page_fault+0x54/0x233 arch/x86/mm/fault.c:1530
 page_fault+0x34/0x40 arch/x86/entry/entry_64.S:1202

write to 0xffff88821878b158 of 8 bytes by task 10481 on cpu 1:
 percpu_counter_add_batch+0xca/0x150 lib/percpu_counter.c:91
 percpu_counter_add include/linux/percpu_counter.h:55 [inline]
 percpu_counter_sub include/linux/percpu_counter.h:189 [inline]
 ext4_mb_mark_diskspace_used+0x8e2/0x9d0 fs/ext4/mballoc.c:3016
 ext4_mb_new_blocks+0x6fc/0x1de0 fs/ext4/mballoc.c:4556
 ext4_ext_map_blocks+0x1c1e/0x2330 fs/ext4/extents.c:4460
 ext4_map_blocks+0x23a/0xfa0 fs/ext4/inode.c:640
 ext4_alloc_file_blocks+0x1bd/0x6c0 fs/ext4/extents.c:4649
 ext4_fallocate+0x507/0x13d0 fs/ext4/extents.c:4934
 vfs_fallocate+0x3be/0x650 fs/open.c:309
 ioctl_preallocate+0x126/0x180 fs/ioctl.c:490
 file_ioctl fs/ioctl.c:506 [inline]
 do_vfs_ioctl+0xbd2/0xc60 fs/ioctl.c:696
 ksys_ioctl+0xbd/0xe0 fs/ioctl.c:713
 __do_sys_ioctl fs/ioctl.c:720 [inline]
 __se_sys_ioctl fs/ioctl.c:718 [inline]
 __x64_sys_ioctl+0x4c/0x60 fs/ioctl.c:718
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 10481 Comm: syz-executor.3 Not tainted 5.4.0-rc6+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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