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BUG: KCSAN: data-race in __mpage_writepage / generic_write_end

write to 0xffff88811ec731d8 of 8 bytes by task 12355 on cpu 1:
 i_size_write include/linux/fs.h:876 [inline]
 generic_write_end+0x99/0x250 fs/buffer.c:2186
 fat_write_end+0x4a/0x160 fs/fat/inode.c:249
 pagecache_write_end+0x54/0x70 mm/filemap.c:3233
 cont_expand_zero fs/buffer.c:2401 [inline]
 cont_write_begin+0x23f/0x610 fs/buffer.c:2459
 fat_write_begin+0x61/0xf0 fs/fat/inode.c:235
 pagecache_write_begin+0x54/0x70 mm/filemap.c:3222
 generic_cont_expand_simple+0x6d/0x100 fs/buffer.c:2360
 fat_cont_expand+0x3a/0x160 fs/fat/file.c:222
 fat_fallocate+0x180/0x1d0 fs/fat/file.c:300
 vfs_fallocate+0x43c/0x5d0 fs/open.c:309
 ksys_fallocate fs/open.c:332 [inline]
 __do_sys_fallocate fs/open.c:340 [inline]
 __se_sys_fallocate fs/open.c:338 [inline]
 __x64_sys_fallocate+0x75/0xc0 fs/open.c:338
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88811ec731d8 of 8 bytes by task 12373 on cpu 0:
 i_size_read include/linux/fs.h:854 [inline]
 __mpage_writepage+0x95/0x1070 fs/mpage.c:501
 write_cache_pages+0x4cb/0x840 mm/page-writeback.c:2246
 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:2352
 __filemap_fdatawrite_range+0x19b/0x1d0 mm/filemap.c:422
 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+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 12373 Comm: syz-executor.1 Not tainted 5.9.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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