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BUG: KCSAN: data-race in shmem_add_to_page_cache / shmem_getpage_gfp

read-write to 0xffff888016fa9ec8 of 8 bytes by task 20339 on cpu 1:
 shmem_add_to_page_cache+0x598/0x6a0 mm/shmem.c:718
 shmem_getpage_gfp+0x85a/0x1960 mm/shmem.c:1927
 shmem_fault+0x16c/0x450 mm/shmem.c:2114
 __do_fault mm/memory.c:3625 [inline]
 do_shared_fault+0xc3/0x7a0 mm/memory.c:4073
 do_fault mm/memory.c:4151 [inline]
 handle_pte_fault mm/memory.c:4387 [inline]
 __handle_mm_fault mm/memory.c:4522 [inline]
 handle_mm_fault+0x1285/0x17b0 mm/memory.c:4620
 faultin_page mm/gup.c:851 [inline]
 __get_user_pages+0xa32/0xff0 mm/gup.c:1070
 __get_user_pages_locked mm/gup.c:1256 [inline]
 __get_user_pages_remote+0x4e6/0x6e0 mm/gup.c:1723
 pin_user_pages_remote+0x58/0x80 mm/gup.c:2791
 process_vm_rw_single_vec+0x178/0x460 mm/process_vm_access.c:106
 process_vm_rw_core mm/process_vm_access.c:216 [inline]
 process_vm_rw+0x3a3/0x540 mm/process_vm_access.c:284
 __do_sys_process_vm_writev mm/process_vm_access.c:304 [inline]
 __se_sys_process_vm_writev mm/process_vm_access.c:299 [inline]
 __x64_sys_process_vm_writev+0x76/0x90 mm/process_vm_access.c:299
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888016fa9ec8 of 8 bytes by task 20322 on cpu 0:
 shmem_recalc_inode mm/shmem.c:367 [inline]
 shmem_getpage_gfp+0xb9a/0x1960 mm/shmem.c:1937
 shmem_fault+0x16c/0x450 mm/shmem.c:2114
 __do_fault mm/memory.c:3625 [inline]
 do_shared_fault+0xc3/0x7a0 mm/memory.c:4073
 do_fault mm/memory.c:4151 [inline]
 handle_pte_fault mm/memory.c:4387 [inline]
 __handle_mm_fault mm/memory.c:4522 [inline]
 handle_mm_fault+0x1285/0x17b0 mm/memory.c:4620
 faultin_page mm/gup.c:851 [inline]
 __get_user_pages+0xa32/0xff0 mm/gup.c:1070
 __get_user_pages_locked mm/gup.c:1256 [inline]
 __get_user_pages_remote+0x4e6/0x6e0 mm/gup.c:1723
 pin_user_pages_remote+0x58/0x80 mm/gup.c:2791
 process_vm_rw_single_vec+0x178/0x460 mm/process_vm_access.c:106
 process_vm_rw_core mm/process_vm_access.c:216 [inline]
 process_vm_rw+0x3a3/0x540 mm/process_vm_access.c:284
 __do_sys_process_vm_writev mm/process_vm_access.c:304 [inline]
 __se_sys_process_vm_writev mm/process_vm_access.c:299 [inline]
 __x64_sys_process_vm_writev+0x76/0x90 mm/process_vm_access.c:299
 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: 0 PID: 20322 Comm: syz-executor.5 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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