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BUG: KCSAN: data-race in shmem_add_to_page_cache / shmem_getpage_gfp

write to 0xffff8881257dff50 of 8 bytes by task 6377 on cpu 1:
 shmem_add_to_page_cache+0x52d/0x650 mm/shmem.c:718
 shmem_getpage_gfp+0xbdc/0x1410 mm/shmem.c:1929
 shmem_fault+0x16d/0x500 mm/shmem.c:2116
 __do_fault mm/memory.c:3678 [inline]
 do_read_fault mm/memory.c:3980 [inline]
 do_fault mm/memory.c:4108 [inline]
 handle_pte_fault mm/memory.c:4367 [inline]
 __handle_mm_fault mm/memory.c:4502 [inline]
 handle_mm_fault+0xe8d/0x1a70 mm/memory.c:4600
 faultin_page mm/gup.c:908 [inline]
 __get_user_pages+0x947/0xee0 mm/gup.c:1127
 populate_vma_page_range mm/gup.c:1460 [inline]
 __mm_populate+0x24d/0x380 mm/gup.c:1508
 mm_populate include/linux/mm.h:2621 [inline]
 __do_sys_mremap+0xb8f/0xc20 mm/mremap.c:967
 __se_sys_mremap mm/mremap.c:816 [inline]
 __x64_sys_mremap+0x63/0x70 mm/mremap.c:816
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff8881257dff50 of 8 bytes by task 6373 on cpu 0:
 shmem_recalc_inode mm/shmem.c:367 [inline]
 shmem_getpage_gfp+0x1017/0x1410 mm/shmem.c:1939
 shmem_fault+0x16d/0x500 mm/shmem.c:2116
 __do_fault mm/memory.c:3678 [inline]
 do_read_fault mm/memory.c:3980 [inline]
 do_fault mm/memory.c:4108 [inline]
 handle_pte_fault mm/memory.c:4367 [inline]
 __handle_mm_fault mm/memory.c:4502 [inline]
 handle_mm_fault+0xe8d/0x1a70 mm/memory.c:4600
 faultin_page mm/gup.c:908 [inline]
 __get_user_pages+0x947/0xee0 mm/gup.c:1127
 populate_vma_page_range mm/gup.c:1460 [inline]
 __mm_populate+0x24d/0x380 mm/gup.c:1508
 mm_populate include/linux/mm.h:2621 [inline]
 vm_mmap_pgoff+0x145/0x1d0 mm/util.c:524
 ksys_mmap_pgoff+0xe1/0x380 mm/mmap.c:1638
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 6373 Comm: syz-executor.1 Not tainted 5.13.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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