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BUG: KCSAN: data-race in __mpage_writepage / generic_write_end

write to 0xffff8881145aacb8 of 8 bytes by task 19301 on cpu 0:
 i_size_write include/linux/fs.h:896 [inline]
 generic_write_end+0x99/0x220 fs/buffer.c:2187
 fat_write_end+0x4a/0x160 fs/fat/inode.c:249
 generic_perform_write+0x23b/0x390 mm/filemap.c:3310
 __generic_file_write_iter+0x154/0x330 mm/filemap.c:3428
 generic_file_write_iter+0x2f3/0x3e0 mm/filemap.c:3460
 call_write_iter include/linux/fs.h:1907 [inline]
 new_sync_write fs/read_write.c:484 [inline]
 __vfs_write+0x2d8/0x340 fs/read_write.c:497
 vfs_write+0x19b/0x350 fs/read_write.c:559
 ksys_write+0xce/0x180 fs/read_write.c:612
 __do_sys_write fs/read_write.c:624 [inline]
 __se_sys_write fs/read_write.c:621 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:621
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8881145aacb8 of 8 bytes by task 19309 on cpu 1:
 i_size_read include/linux/fs.h:874 [inline]
 __mpage_writepage+0x95/0x1070 fs/mpage.c:501
 write_cache_pages+0x4cb/0x840 mm/page-writeback.c:2248
 mpage_writepages+0x6d/0x120 fs/mpage.c:714
 fat_writepages+0x20/0x30 fs/fat/inode.c:205
 do_writepages+0x7b/0x150 mm/page-writeback.c:2354
 __filemap_fdatawrite_range+0x19b/0x1d0 mm/filemap.c:421
 generic_fadvise+0x316/0x420 mm/fadvise.c:113
 vfs_fadvise mm/fadvise.c:185 [inline]
 ksys_fadvise64_64 mm/fadvise.c:199 [inline]
 __do_sys_fadvise64 mm/fadvise.c:214 [inline]
 __se_sys_fadvise64 mm/fadvise.c:212 [inline]
 __x64_sys_fadvise64+0xc5/0x100 mm/fadvise.c:212
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 19309 Comm: syz-executor.1 Not tainted 5.8.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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