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BUG: KCSAN: data-race in __add_to_page_cache_locked / ext4_writepages

write to 0xffff888128b69e00 of 8 bytes by task 32008 on cpu 0:
 __add_to_page_cache_locked+0x28a/0x770 mm/filemap.c:868
 add_to_page_cache_lru+0xc7/0x260 mm/filemap.c:921
 pagecache_get_page+0x2b0/0x700 mm/filemap.c:1656
 grab_cache_page_write_begin+0x56/0x80 mm/filemap.c:3252
 ext4_da_write_begin+0x1b4/0x860 fs/ext4/inode.c:2964
 generic_perform_write+0x13a/0x320 mm/filemap.c:3302
 ext4_buffered_write_iter+0x14e/0x280 fs/ext4/file.c:270
 ext4_file_write_iter+0xf4/0xd30 fs/ext4/file.c:642
 call_write_iter include/linux/fs.h:1907 [inline]
 new_sync_write+0x303/0x400 fs/read_write.c:484
 __vfs_write+0x9e/0xb0 fs/read_write.c:497
 vfs_write fs/read_write.c:559 [inline]
 vfs_write+0x189/0x380 fs/read_write.c:543
 ksys_write+0xc5/0x1a0 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+0x49/0x60 fs/read_write.c:621
 do_syscall_64+0xc7/0x3b0 arch/x86/entry/common.c:295
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888128b69e00 of 8 bytes by task 32006 on cpu 1:
 ext4_writepages+0x186/0x1e10 fs/ext4/inode.c:2637
 do_writepages+0x6b/0x170 mm/page-writeback.c:2344
 __filemap_fdatawrite_range+0x1bb/0x220 mm/filemap.c:421
 __filemap_fdatawrite mm/filemap.c:429 [inline]
 filemap_flush+0x2a/0x40 mm/filemap.c:456
 ext4_alloc_da_blocks+0x72/0x170 fs/ext4/inode.c:3144
 ext4_release_file+0x107/0x200 fs/ext4/file.c:142
 __fput+0x1e9/0x500 fs/file_table.c:280
 ____fput+0x1b/0x30 fs/file_table.c:313
 task_work_run+0xba/0x120 kernel/task_work.c:123
 tracehook_notify_resume include/linux/tracehook.h:188 [inline]
 exit_to_usermode_loop+0x2ae/0x2c0 arch/x86/entry/common.c:165
 prepare_exit_to_usermode arch/x86/entry/common.c:196 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:279 [inline]
 do_syscall_64+0x38b/0x3b0 arch/x86/entry/common.c:305
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 32006 Comm: syz-executor.1 Not tainted 5.7.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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