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BUG: KCSAN: data-race in __add_to_page_cache_locked / ext4_writepages

write to 0xffff88810ebd7c58 of 8 bytes by task 19517 on cpu 0:
 __add_to_page_cache_locked+0x3ad/0x6e0 mm/filemap.c:887
 add_to_page_cache_lru+0xa0/0x1b0 mm/filemap.c:940
 pagecache_get_page+0x6a0/0xa30 mm/filemap.c:1808
 grab_cache_page_write_begin+0x3f/0x70 mm/filemap.c:3525
 ext4_da_write_begin+0x34d/0xaf0 fs/ext4/inode.c:2986
 generic_perform_write+0x196/0x3a0 mm/filemap.c:3575
 ext4_buffered_write_iter+0x2e5/0x3e0 fs/ext4/file.c:269
 ext4_file_write_iter+0x48a/0x10b0 fs/ext4/file.c:502
 call_write_iter include/linux/fs.h:1977 [inline]
 new_sync_write fs/read_write.c:518 [inline]
 vfs_write+0x6f9/0x7e0 fs/read_write.c:605
 ksys_write+0xce/0x180 fs/read_write.c:658
 __do_sys_write fs/read_write.c:670 [inline]
 __se_sys_write fs/read_write.c:667 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:667
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88810ebd7c58 of 8 bytes by task 19514 on cpu 1:
 ext4_writepages+0x136/0x1e70 fs/ext4/inode.c:2670
 do_writepages+0x7b/0x150 mm/page-writeback.c:2352
 __filemap_fdatawrite_range+0x19d/0x1d0 mm/filemap.c:424
 __filemap_fdatawrite mm/filemap.c:432 [inline]
 filemap_flush+0x1f/0x30 mm/filemap.c:459
 ext4_alloc_da_blocks+0x4a/0x100 fs/ext4/inode.c:3166
 ext4_release_file+0x5b/0x1d0 fs/ext4/file.c:141
 __fput+0x263/0x4f0 fs/file_table.c:280
 ____fput+0x11/0x20 fs/file_table.c:313
 task_work_run+0x8e/0x110 kernel/task_work.c:140
 tracehook_notify_resume include/linux/tracehook.h:189 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:174 [inline]
 exit_to_user_mode_prepare+0x17c/0x1b0 kernel/entry/common.c:208
 __syscall_exit_to_user_mode_work kernel/entry/common.c:290 [inline]
 syscall_exit_to_user_mode+0x20/0x40 kernel/entry/common.c:301
 do_syscall_64+0x45/0x80 arch/x86/entry/common.c:56
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 19514 Comm: syz-executor.2 Not tainted 5.12.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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