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BUG: KCSAN: data-race in generic_write_end / mpage_submit_page

write to 0xffff8880bb45f6f8 of 8 bytes by task 10799 on cpu 0:
 i_size_write include/linux/fs.h:896 [inline]
 generic_write_end+0xac/0x2a0 fs/buffer.c:2187
 ext4_da_write_end+0x55e/0x6d0 fs/ext4/inode.c:3092
 generic_perform_write+0x233/0x380 mm/filemap.c:3310
 ext4_buffered_write_iter+0x2b1/0x390 fs/ext4/file.c:270
 ext4_file_write_iter+0x4a3/0xfd0 fs/ext4/file.c:498
 call_write_iter include/linux/fs.h:1907 [inline]
 new_sync_write fs/read_write.c:484 [inline]
 __vfs_write+0x2d8/0x340 fs/read_write.c:497
 vfs_write+0x19b/0x350 fs/read_write.c:559
 ksys_write+0xce/0x180 fs/read_write.c:612
 __do_sys_write fs/read_write.c:624 [inline]
 __se_sys_write fs/read_write.c:621 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:621
 do_syscall_64+0x71/0x120 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880bb45f6f8 of 8 bytes by task 10806 on cpu 1:
 i_size_read include/linux/fs.h:874 [inline]
 mpage_submit_page+0x67/0x170 fs/ext4/inode.c:2076
 mpage_map_and_submit_buffers fs/ext4/inode.c:2330 [inline]
 mpage_map_and_submit_extent fs/ext4/inode.c:2469 [inline]
 ext4_writepages+0x106b/0x1cb0 fs/ext4/inode.c:2782
 do_writepages+0x7b/0x150 mm/page-writeback.c:2354
 __filemap_fdatawrite_range+0x19b/0x1d0 mm/filemap.c:421
 filemap_write_and_wait_range+0x8b/0x230 mm/filemap.c:654
 filemap_write_and_wait include/linux/fs.h:2765 [inline]
 ext4_bmap+0x102/0x1f0 fs/ext4/inode.c:3182
 bmap+0x56/0x80 fs/inode.c:1703
 ioctl_fibmap fs/ioctl.c:74 [inline]
 file_ioctl fs/ioctl.c:531 [inline]
 do_vfs_ioctl+0xd44/0xec0 fs/ioctl.c:732
 ksys_ioctl fs/ioctl.c:751 [inline]
 __do_sys_ioctl fs/ioctl.c:762 [inline]
 __se_sys_ioctl+0x81/0x130 fs/ioctl.c:760
 __x64_sys_ioctl+0x3f/0x50 fs/ioctl.c:760
 do_syscall_64+0x71/0x120 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 10806 Comm: syz-executor.2 Not tainted 5.8.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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