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BUG: KCSAN: data-race in ext4_alloc_file_blocks / mpage_process_page_bufs

write to 0xffff888100ea67f8 of 8 bytes by task 901 on cpu 1:
 i_size_write include/linux/fs.h:866 [inline]
 ext4_update_inode_size fs/ext4/ext4.h:3109 [inline]
 ext4_alloc_file_blocks+0x325/0x6e0 fs/ext4/extents.c:4450
 ext4_zero_range+0x2c6/0xa30 fs/ext4/extents.c:4544
 ext4_fallocate+0x214/0x560 fs/ext4/extents.c:4672
 vfs_fallocate+0x431/0x580 fs/open.c:309
 ksys_fallocate fs/open.c:332 [inline]
 __do_sys_fallocate fs/open.c:340 [inline]
 __se_sys_fallocate fs/open.c:338 [inline]
 __x64_sys_fallocate+0x75/0xc0 fs/open.c:338
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888100ea67f8 of 8 bytes by task 906 on cpu 0:
 i_size_read include/linux/fs.h:844 [inline]
 mpage_process_page_bufs+0x34/0x600 fs/ext4/inode.c:2174
 mpage_prepare_extent_to_map+0x4ec/0x650 fs/ext4/inode.c:2608
 ext4_writepages+0x97d/0x1ef0 fs/ext4/inode.c:2780
 do_writepages+0x7b/0x150 mm/page-writeback.c:2354
 __filemap_fdatawrite_range+0x19b/0x1d0 mm/filemap.c:421
 __filemap_fdatawrite mm/filemap.c:429 [inline]
 filemap_flush+0x1f/0x30 mm/filemap.c:456
 ext4_alloc_da_blocks+0x4c/0x130 fs/ext4/inode.c:3146
 ext4_release_file+0x5b/0x1c0 fs/ext4/file.c:142
 __fput+0x1df/0x460 fs/file_table.c:281
 ____fput+0x11/0x20 fs/file_table.c:314
 task_work_run+0x8e/0x110 kernel/task_work.c:135
 get_signal+0x3f6/0x1650 kernel/signal.c:2547
 do_signal+0x25/0x270 arch/x86/kernel/signal.c:810
 exit_to_usermode_loop arch/x86/entry/common.c:235 [inline]
 __prepare_exit_to_usermode+0x127/0x230 arch/x86/entry/common.c:269
 __syscall_return_slowpath+0x47/0x60 arch/x86/entry/common.c:352
 do_syscall_64+0x5d/0xb0 arch/x86/entry/common.c:393
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 906 Comm: syz-executor.2 Not tainted 5.8.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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