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BUG: KCSAN: data-race in filemap_fault / filemap_fault

write to 0xffff88812b2ecdac of 4 bytes by task 3391 on cpu 1:
 do_sync_mmap_readahead mm/filemap.c:2389 [inline]
 filemap_fault+0x411/0x1150 mm/filemap.c:2491
 ext4_filemap_fault+0x4b/0x60 fs/ext4/inode.c:6060
 __do_fault mm/memory.c:3453 [inline]
 do_read_fault+0x41f/0x720 mm/memory.c:3846
 do_fault mm/memory.c:3974 [inline]
 handle_pte_fault mm/memory.c:4214 [inline]
 __handle_mm_fault mm/memory.c:4346 [inline]
 handle_mm_fault+0x12ba/0x18a0 mm/memory.c:4383
 do_user_addr_fault+0x393/0x810 arch/x86/mm/fault.c:1295
 handle_page_fault arch/x86/mm/fault.c:1365 [inline]
 exc_page_fault+0xb8/0x330 arch/x86/mm/fault.c:1418
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:565

read to 0xffff88812b2ecdac of 4 bytes by task 3387 on cpu 0:
 do_sync_mmap_readahead mm/filemap.c:2388 [inline]
 filemap_fault+0x3e6/0x1150 mm/filemap.c:2491
 ext4_filemap_fault+0x4b/0x60 fs/ext4/inode.c:6060
 __do_fault mm/memory.c:3453 [inline]
 do_read_fault+0x41f/0x720 mm/memory.c:3846
 do_fault mm/memory.c:3974 [inline]
 handle_pte_fault mm/memory.c:4214 [inline]
 __handle_mm_fault mm/memory.c:4346 [inline]
 handle_mm_fault+0x12ba/0x18a0 mm/memory.c:4383
 do_user_addr_fault+0x393/0x810 arch/x86/mm/fault.c:1295
 handle_page_fault arch/x86/mm/fault.c:1365 [inline]
 exc_page_fault+0xb8/0x330 arch/x86/mm/fault.c:1418
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:565
 fault_in_pages_readable arch/x86/include/asm/smap.h:53 [inline]
 iov_iter_fault_in_readable+0x14f/0x390 lib/iov_iter.c:431
 generic_perform_write+0xd2/0x380 mm/filemap.c:3289
 ext4_buffered_write_iter+0x2b1/0x390 fs/ext4/file.c:270
 ext4_file_write_iter+0x4a3/0xfd0 fs/ext4/file.c:498
 call_write_iter include/linux/fs.h:1907 [inline]
 new_sync_write fs/read_write.c:484 [inline]
 __vfs_write+0x2d8/0x340 fs/read_write.c:497
 vfs_write+0x19b/0x350 fs/read_write.c:559
 ksys_write+0xce/0x180 fs/read_write.c:612
 __do_sys_write fs/read_write.c:624 [inline]
 __se_sys_write fs/read_write.c:621 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:621
 do_syscall_64+0x71/0x120 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 3387 Comm: syz-executor.0 Not tainted 5.8.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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