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BUG: KCSAN: data-race in shmem_add_to_page_cache / shmem_getpage_gfp

write to 0xffff888109580388 of 8 bytes by task 920 on cpu 0:
 shmem_add_to_page_cache+0x5d7/0x900 mm/shmem.c:718
 shmem_getpage_gfp+0x821/0x19c0 mm/shmem.c:1920
 shmem_fault+0x16c/0x430 mm/shmem.c:2107
 __do_fault mm/memory.c:3639 [inline]
 do_read_fault+0x41f/0x760 mm/memory.c:4032
 do_fault mm/memory.c:4160 [inline]
 handle_pte_fault mm/memory.c:4400 [inline]
 __handle_mm_fault mm/memory.c:4535 [inline]
 handle_mm_fault+0x14e9/0x1af0 mm/memory.c:4633
 faultin_page mm/gup.c:878 [inline]
 __get_user_pages+0xa3d/0x1010 mm/gup.c:1090
 populate_vma_page_range mm/gup.c:1423 [inline]
 __mm_populate+0x24d/0x340 mm/gup.c:1471
 mm_populate include/linux/mm.h:2566 [inline]
 vm_mmap_pgoff+0x133/0x170 mm/util.c:511
 ksys_mmap_pgoff+0xe1/0x380 mm/mmap.c:1596
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888109580388 of 8 bytes by task 916 on cpu 1:
 shmem_recalc_inode mm/shmem.c:367 [inline]
 shmem_getpage_gfp+0xb7c/0x19c0 mm/shmem.c:1930
 shmem_fault+0x16c/0x430 mm/shmem.c:2107
 __do_fault mm/memory.c:3639 [inline]
 do_read_fault+0x41f/0x760 mm/memory.c:4032
 do_fault mm/memory.c:4160 [inline]
 handle_pte_fault mm/memory.c:4400 [inline]
 __handle_mm_fault mm/memory.c:4535 [inline]
 handle_mm_fault+0x14e9/0x1af0 mm/memory.c:4633
 faultin_page mm/gup.c:878 [inline]
 __get_user_pages+0xa3d/0x1010 mm/gup.c:1090
 __get_user_pages_locked mm/gup.c:1276 [inline]
 get_user_pages_unlocked+0x148/0x570 mm/gup.c:1996
 __gup_longterm_unlocked mm/gup.c:2646 [inline]
 internal_get_user_pages_fast+0xfc2/0x1240 mm/gup.c:2707
 get_user_pages_fast+0x46/0x60 mm/gup.c:2803
 iov_iter_get_pages+0x1a3/0x850 lib/iov_iter.c:1324
 af_alg_make_sg+0x45/0x250 crypto/af_alg.c:394
 hash_sendmsg+0x162/0x650 crypto/algif_hash.c:94
 sock_sendmsg_nosec net/socket.c:651 [inline]
 sock_sendmsg net/socket.c:671 [inline]
 __sys_sendto+0x2ae/0x380 net/socket.c:1992
 __do_sys_sendto net/socket.c:2004 [inline]
 __se_sys_sendto net/socket.c:2000 [inline]
 __x64_sys_sendto+0x74/0x90 net/socket.c:2000
 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: 916 Comm: syz-executor.0 Not tainted 5.9.0-rc8-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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