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BUG: KCSAN: data-race in __add_to_page_cache_locked / ext4_writepages

read to 0xffff88812582e2f0 of 8 bytes by task 406 on cpu 1:
 ext4_writepages+0x188/0x2300 fs/ext4/inode.c:2736
 do_writepages+0x6b/0x170 mm/page-writeback.c:2344
 __writeback_single_inode+0xaf/0x8e0 fs/fs-writeback.c:1449
 writeback_sb_inodes+0x4ba/0xa50 fs/fs-writeback.c:1713
 __writeback_inodes_wb+0xe8/0x1b0 fs/fs-writeback.c:1782
 wb_writeback+0x557/0x6a0 fs/fs-writeback.c:1891
 wb_check_background_flush fs/fs-writeback.c:1959 [inline]
 wb_do_writeback fs/fs-writeback.c:2047 [inline]
 wb_workfn+0x696/0x970 fs/fs-writeback.c:2075
 process_one_work+0x3d4/0x890 kernel/workqueue.c:2269
 worker_thread+0xa0/0x800 kernel/workqueue.c:2415
 kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

write to 0xffff88812582e2f0 of 8 bytes by task 30293 on cpu 0:
 __add_to_page_cache_locked+0x298/0x780 mm/filemap.c:889
 add_to_page_cache_lru+0xe2/0x2d0 mm/filemap.c:942
 pagecache_get_page+0x2ab/0x760 mm/filemap.c:1681
 grab_cache_page_write_begin+0x5d/0x90 mm/filemap.c:3254
 ext4_da_write_begin+0x175/0x7e0 fs/ext4/inode.c:3063
 generic_perform_write+0x136/0x320 mm/filemap.c:3304
 __generic_file_write_iter+0x251/0x380 mm/filemap.c:3433
 ext4_file_write_iter+0x1bd/0xa00 fs/ext4/file.c:268
 call_write_iter include/linux/fs.h:1895 [inline]
 new_sync_write+0x388/0x4a0 fs/read_write.c:483
 __vfs_write+0xb1/0xc0 fs/read_write.c:496
 vfs_write fs/read_write.c:558 [inline]
 vfs_write+0x18a/0x390 fs/read_write.c:542
 ksys_write+0xd5/0x1b0 fs/read_write.c:611
 __do_sys_write fs/read_write.c:623 [inline]
 __se_sys_write fs/read_write.c:620 [inline]
 __x64_sys_write+0x4c/0x60 fs/read_write.c:620

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 30293 Comm: syz-executor.1 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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