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BUG: KCSAN: data-race in filemap_fault / filemap_map_pages

write to 0xffff88812a9a20ac of 4 bytes by task 5076 on cpu 1:
 do_sync_mmap_readahead mm/filemap.c:2487 [inline]
 filemap_fault+0x42f/0x11e0 mm/filemap.c:2589
 ext4_filemap_fault+0x4b/0x60 fs/ext4/inode.c:6060
 __do_fault mm/memory.c:3467 [inline]
 do_read_fault+0x41f/0x730 mm/memory.c:3860
 do_fault mm/memory.c:3988 [inline]
 handle_pte_fault mm/memory.c:4228 [inline]
 __handle_mm_fault mm/memory.c:4360 [inline]
 handle_mm_fault+0x136c/0x1950 mm/memory.c:4397
 do_user_addr_fault+0x393/0x810 arch/x86/mm/fault.c:1294
 handle_page_fault arch/x86/mm/fault.c:1364 [inline]
 exc_page_fault+0xb8/0x330 arch/x86/mm/fault.c:1417
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:538

read to 0xffff88812a9a20ac of 4 bytes by task 15349 on cpu 0:
 filemap_map_pages+0x68d/0x990 mm/filemap.c:2727
 do_fault_around mm/memory.c:3821 [inline]
 do_read_fault+0x22f/0x730 mm/memory.c:3855
 do_fault mm/memory.c:3988 [inline]
 handle_pte_fault mm/memory.c:4228 [inline]
 __handle_mm_fault mm/memory.c:4360 [inline]
 handle_mm_fault+0x136c/0x1950 mm/memory.c:4397
 do_user_addr_fault+0x393/0x810 arch/x86/mm/fault.c:1294
 handle_page_fault arch/x86/mm/fault.c:1364 [inline]
 exc_page_fault+0xb8/0x330 arch/x86/mm/fault.c:1417
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:538

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 15349 Comm: systemd-udevd Not tainted 5.8.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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