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BUG: KCSAN: data-race in block_page_mkwrite / generic_write_end

write to 0xffff888131cda350 of 8 bytes by task 12443 on cpu 0:
 i_size_write include/linux/fs.h:870 [inline]
 generic_write_end+0x98/0x250 fs/buffer.c:2186
 ext4_da_write_end+0x59b/0x760 fs/ext4/inode.c:3111
 generic_perform_write+0x23e/0x3a0 mm/filemap.c:3419
 ext4_buffered_write_iter+0x2e5/0x3d0 fs/ext4/file.c:270
 ext4_file_write_iter+0x45e/0x1090 fs/ext4/file.c:503
 call_write_iter include/linux/fs.h:1901 [inline]
 new_sync_write fs/read_write.c:518 [inline]
 vfs_write+0x6d4/0x7c0 fs/read_write.c:605
 ksys_write+0xce/0x180 fs/read_write.c:658
 __do_sys_write fs/read_write.c:670 [inline]
 __se_sys_write fs/read_write.c:667 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:667
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888131cda350 of 8 bytes by task 12448 on cpu 1:
 i_size_read include/linux/fs.h:848 [inline]
 block_page_mkwrite+0xa9/0x2e0 fs/buffer.c:2509
 ext4_page_mkwrite+0x736/0xf40 fs/ext4/inode.c:6092
 do_page_mkwrite mm/memory.c:2714 [inline]
 wp_page_shared+0x2a7/0x7c0 mm/memory.c:3045
 do_wp_page+0x676/0x6c0 mm/memory.c:3138
 handle_pte_fault mm/memory.c:4405 [inline]
 __handle_mm_fault mm/memory.c:4522 [inline]
 handle_mm_fault+0x1407/0x17b0 mm/memory.c:4620
 do_user_addr_fault+0x433/0x830 arch/x86/mm/fault.c:1393
 handle_page_fault arch/x86/mm/fault.c:1450 [inline]
 exc_page_fault+0x94/0x2a0 arch/x86/mm/fault.c:1506
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:580
 __clear_user arch/x86/lib/usercopy_64.c:24 [inline]
 clear_user+0x60/0xa0 arch/x86/lib/usercopy_64.c:54
 iov_iter_zero+0x15c/0x9e0 lib/iov_iter.c:982
 read_iter_zero+0x61/0x150 drivers/char/mem.c:718
 call_read_iter include/linux/fs.h:1895 [inline]
 do_iter_readv_writev+0x2e8/0x360 fs/read_write.c:738
 do_iter_read+0x109/0x4b0 fs/read_write.c:801
 vfs_readv fs/read_write.c:921 [inline]
 do_readv+0x13d/0x280 fs/read_write.c:958
 __do_sys_readv fs/read_write.c:1049 [inline]
 __se_sys_readv fs/read_write.c:1046 [inline]
 __x64_sys_readv+0x41/0x50 fs/read_write.c:1046
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 12448 Comm: syz-executor.4 Not tainted 5.11.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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