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BUG: KCSAN: data-race in shmem_add_to_page_cache / shmem_getpage_gfp

read-write to 0xffff88810c86e518 of 8 bytes by task 12699 on cpu 0:
 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_read_fault+0x403/0x760 mm/memory.c:4019
 do_fault mm/memory.c:4147 [inline]
 handle_pte_fault mm/memory.c:4387 [inline]
 __handle_mm_fault mm/memory.c:4522 [inline]
 handle_mm_fault+0xff1/0x17b0 mm/memory.c:4620
 faultin_page mm/gup.c:851 [inline]
 __get_user_pages+0xa32/0xff0 mm/gup.c:1070
 populate_vma_page_range mm/gup.c:1403 [inline]
 __mm_populate+0x24d/0x380 mm/gup.c:1451
 mm_populate include/linux/mm.h:2591 [inline]
 vm_mmap_pgoff+0x14c/0x1d0 mm/util.c:524
 ksys_mmap_pgoff+0xe1/0x380 mm/mmap.c:1634
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88810c86e518 of 8 bytes by task 12689 on cpu 1:
 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_read_fault+0x403/0x760 mm/memory.c:4019
 do_fault mm/memory.c:4147 [inline]
 handle_pte_fault mm/memory.c:4387 [inline]
 __handle_mm_fault mm/memory.c:4522 [inline]
 handle_mm_fault+0xff1/0x17b0 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_unlocked+0x135/0x5f0 mm/gup.c:1938
 __gup_longterm_unlocked+0x4e/0x220 mm/gup.c:2510
 internal_get_user_pages_fast+0x7f4/0x900 mm/gup.c:2598
 get_user_pages_fast+0x5d/0x80 mm/gup.c:2690
 iov_iter_get_pages+0x1b4/0x880 lib/iov_iter.c:1328
 af_alg_make_sg+0x49/0x260 crypto/af_alg.c:410
 hash_sendmsg+0x162/0x650 crypto/algif_hash.c:94
 sock_sendmsg_nosec net/socket.c:652 [inline]
 sock_sendmsg net/socket.c:672 [inline]
 __sys_sendto+0x2a8/0x370 net/socket.c:1975
 __do_sys_sendto net/socket.c:1987 [inline]
 __se_sys_sendto net/socket.c:1983 [inline]
 __x64_sys_sendto+0x74/0x90 net/socket.c:1983
 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: 12689 Comm: syz-executor.1 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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