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BUG: KCSAN: data-race in __add_to_page_cache_locked / ext4_writepages

write to 0xffff8880bb03b738 of 8 bytes by task 8873 on cpu 1:
 __add_to_page_cache_locked+0x298/0x780 mm/filemap.c:890
 add_to_page_cache_lru+0xc4/0x260 mm/filemap.c:943
 pagecache_get_page+0x2ac/0x6f0 mm/filemap.c:1682
 grab_cache_page_write_begin+0x5d/0x90 mm/filemap.c:3255
 ext4_da_write_begin+0x1a3/0x810 fs/ext4/inode.c:3063
 generic_perform_write+0x136/0x320 mm/filemap.c:3305
 __generic_file_write_iter+0x251/0x380 mm/filemap.c:3434
 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
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880bb03b738 of 8 bytes by task 8875 on cpu 0:
 ext4_writepages+0x188/0x2300 fs/ext4/inode.c:2736
 do_writepages+0x6b/0x170 mm/page-writeback.c:2344
 __filemap_fdatawrite_range+0x1c5/0x230 mm/filemap.c:421
 generic_fadvise+0x4a0/0x4b0 mm/fadvise.c:115
 vfs_fadvise+0x92/0xa0 mm/fadvise.c:187
 ksys_fadvise64_64+0x61/0xa0 mm/fadvise.c:201
 __do_sys_fadvise64 mm/fadvise.c:216 [inline]
 __se_sys_fadvise64 mm/fadvise.c:214 [inline]
 __x64_sys_fadvise64+0x5e/0x80 mm/fadvise.c:214
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 8875 Comm: syz-executor.3 Not tainted 5.4.0-rc7+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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