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BUG: KCSAN: data-race in __add_to_page_cache_locked / ext4_writepages

write to 0xffff88813b8ef458 of 8 bytes by task 20430 on cpu 1:
 __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 0xffff88813b8ef458 of 8 bytes by task 20434 on cpu 0:
 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_write_and_wait_range+0x8b/0x2a0 mm/filemap.c:657
 filemap_write_and_wait include/linux/fs.h:2753 [inline]
 ext4_bmap+0x124/0x220 fs/ext4/inode.c:3202
 bmap+0x56/0x80 fs/inode.c:1705
 ioctl_fibmap fs/ioctl.c:74 [inline]
 file_ioctl fs/ioctl.c:531 [inline]
 do_vfs_ioctl+0xd38/0xec0 fs/ioctl.c:732
 __do_sys_ioctl fs/ioctl.c:751 [inline]
 __se_sys_ioctl+0x83/0x140 fs/ioctl.c:739
 __x64_sys_ioctl+0x3f/0x50 fs/ioctl.c:739
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 20434 Comm: syz-executor.0 Not tainted 5.12.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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