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BUG: KCSAN: data-race in ext4_nonda_switch / percpu_counter_add_batch

write to 0xffff8882187798e0 of 8 bytes by task 7489 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+0x525/0x9d0 fs/ext4/mballoc.c:3010
 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
 ksys_fallocate+0x61/0xa0 fs/open.c:332
 __do_sys_fallocate fs/open.c:340 [inline]
 __se_sys_fallocate fs/open.c:338 [inline]
 __x64_sys_fallocate+0x5e/0x80 fs/open.c:338
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8882187798e0 of 8 bytes by task 7502 on cpu 0:
 ext4_nonda_switch+0x3d/0x140 fs/ext4/inode.c:2988
 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+0x2099/0x2cb0 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

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 7502 Comm: syz-executor.3 Not tainted 5.4.0-rc3+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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