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BUG: KCSAN: data-race in shmem_add_to_page_cache / shmem_recalc_inode

write to 0xffff88809f4a0828 of 8 bytes by task 816 on cpu 1:
 shmem_add_to_page_cache+0x497/0x6b0 mm/shmem.c:643
 shmem_getpage_gfp+0x3fd/0x13c0 mm/shmem.c:1875
 shmem_fault+0x114/0x3f0 mm/shmem.c:2065
 __do_fault+0xae/0x1f0 mm/memory.c:3092
 do_read_fault mm/memory.c:3489 [inline]
 do_fault mm/memory.c:3618 [inline]
 handle_pte_fault mm/memory.c:3849 [inline]
 __handle_mm_fault+0x1fa0/0x2cb0 mm/memory.c:3973
 handle_mm_fault+0x21b/0x530 mm/memory.c:4010
 do_user_addr_fault arch/x86/mm/fault.c:1441 [inline]
 __do_page_fault+0x3fb/0x9e0 arch/x86/mm/fault.c:1506
 do_page_fault+0x54/0x233 arch/x86/mm/fault.c:1530
 page_fault+0x34/0x40 arch/x86/entry/entry_64.S:1202
 fault_in_pages_readable include/linux/pagemap.h:600 [inline]
 iov_iter_fault_in_readable+0x239/0x2b0 lib/iov_iter.c:426
 generic_perform_write+0xc6/0x320 mm/filemap.c:3294
 __generic_file_write_iter+0x251/0x380 mm/filemap.c:3433
 ext4_file_write_iter+0x1bd/0xa00 fs/ext4/file.c:268
 call_write_iter include/linux/fs.h:1895 [inline]
 new_sync_write+0x388/0x4a0 fs/read_write.c:483

read to 0xffff88809f4a0828 of 8 bytes by task 837 on cpu 0:
 shmem_recalc_inode+0x54/0x190 mm/shmem.c:306
 shmem_getpage_gfp+0x552/0x13c0 mm/shmem.c:1889
 shmem_fault+0x114/0x3f0 mm/shmem.c:2065
 __do_fault+0xae/0x1f0 mm/memory.c:3092
 do_read_fault mm/memory.c:3489 [inline]
 do_fault mm/memory.c:3618 [inline]
 handle_pte_fault mm/memory.c:3849 [inline]
 __handle_mm_fault+0x1fa0/0x2cb0 mm/memory.c:3973
 handle_mm_fault+0x21b/0x530 mm/memory.c:4010
 faultin_page mm/gup.c:640 [inline]
 __get_user_pages+0x485/0x1160 mm/gup.c:845
 populate_vma_page_range+0xe6/0x100 mm/gup.c:1223
 __mm_populate+0x168/0x2a0 mm/gup.c:1271
 mm_populate include/linux/mm.h:2362 [inline]
 vm_mmap_pgoff+0x181/0x190 mm/util.c:501
 ksys_mmap_pgoff+0x99/0x420 mm/mmap.c:1629
 __do_sys_mmap arch/x86/kernel/sys_x86_64.c:100 [inline]
 __se_sys_mmap arch/x86/kernel/sys_x86_64.c:91 [inline]
 __x64_sys_mmap+0x91/0xc0 arch/x86/kernel/sys_x86_64.c:91
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 837 Comm: syz-executor.5 Not tainted 5.4.0-rc3+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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