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BUG: KCSAN: data-race in ext4_write_end / mpage_process_page_bufs

write to 0xffff888128d3e1b8 of 8 bytes by task 27323 on cpu 0:
 i_size_write include/linux/fs.h:888 [inline]
 ext4_update_inode_size fs/ext4/ext4.h:3042 [inline]
 ext4_write_end+0x30d/0x7f0 fs/ext4/inode.c:1287
 generic_perform_write+0x1d7/0x320 mm/filemap.c:3298
 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:1901 [inline]
 new_sync_write+0x303/0x400 fs/read_write.c:483
 __vfs_write+0x9e/0xb0 fs/read_write.c:496
 vfs_write fs/read_write.c:558 [inline]
 vfs_write+0x189/0x380 fs/read_write.c:542
 ksys_write+0xc5/0x1a0 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+0x49/0x60 fs/read_write.c:620
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888128d3e1b8 of 8 bytes by task 27330 on cpu 1:
 mpage_process_page_bufs+0x3f/0x400 fs/ext4/inode.c:2166
 mpage_prepare_extent_to_map+0x43b/0x6e0 fs/ext4/inode.c:2597
 ext4_writepages+0xb00/0x1d40 fs/ext4/inode.c:2769
 do_writepages+0x6b/0x170 mm/page-writeback.c:2344
 __filemap_fdatawrite_range+0x1bb/0x220 mm/filemap.c:421
 file_write_and_wait_range+0xfb/0x160 mm/filemap.c:760
 ext4_sync_file+0x26e/0x8b0 fs/ext4/fsync.c:153
 vfs_fsync_range+0x7c/0x150 fs/sync.c:197
 vfs_fsync fs/sync.c:211 [inline]
 do_fsync+0x57/0x90 fs/sync.c:221
 __do_sys_fdatasync fs/sync.c:234 [inline]
 __se_sys_fdatasync fs/sync.c:232 [inline]
 __x64_sys_fdatasync+0x2a/0x40 fs/sync.c:232
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 27330 Comm: syz-executor.3 Not tainted 5.6.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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