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BUG: KCSAN: data-race in shmem_add_to_page_cache / shmem_getpage_gfp

write to 0xffff888020a30e48 of 8 bytes by task 14899 on cpu 1:
 shmem_add_to_page_cache+0x5a0/0x6b0 mm/shmem.c:718
 shmem_getpage_gfp+0x829/0x1930 mm/shmem.c:1927
 shmem_fault+0x16c/0x450 mm/shmem.c:2114
 __do_fault mm/memory.c:3625 [inline]
 do_read_fault+0x3fb/0x720 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+0x100c/0x1810 mm/memory.c:4620
 do_user_addr_fault+0x43e/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
 __get_user_nocheck_1+0x6/0x10 arch/x86/lib/getuser.S:103
 fault_in_pages_readable include/linux/pagemap.h:747 [inline]
 iov_iter_fault_in_readable+0x155/0x3a0 lib/iov_iter.c:438
 generic_perform_write+0xd2/0x3a0 mm/filemap.c:3402
 ext4_buffered_write_iter+0x2e5/0x3e0 fs/ext4/file.c:270
 ext4_file_write_iter+0x47a/0x1060 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+0x6f0/0x7e0 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 0xffff888020a30e48 of 8 bytes by task 14891 on cpu 0:
 shmem_recalc_inode mm/shmem.c:367 [inline]
 shmem_getpage_gfp+0xb5c/0x1930 mm/shmem.c:1937
 shmem_fault+0x16c/0x450 mm/shmem.c:2114
 __do_fault mm/memory.c:3625 [inline]
 do_read_fault+0x3fb/0x720 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+0x100c/0x1810 mm/memory.c:4620
 faultin_page mm/gup.c:851 [inline]
 __get_user_pages+0xa42/0x1000 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

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 14891 Comm: syz-executor.1 Not tainted 5.11.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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