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BUG: KCSAN: data-race in __filemap_add_folio / filemap_write_and_wait_range

read-write to 0xffff88811aa250f0 of 8 bytes by task 26359 on cpu 0:
 __filemap_add_folio+0x66e/0x880 mm/filemap.c:920
 filemap_add_folio+0x1d3/0x350 mm/filemap.c:967
 __filemap_get_folio_mpol+0x32e/0x840 mm/filemap.c:2023
 __filemap_get_folio include/linux/pagemap.h:763 [inline]
 block_write_begin fs/buffer.c:2229 [inline]
 cont_write_begin+0x58b/0x920 fs/buffer.c:2596
 fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
 cont_expand_zero fs/buffer.c:2524 [inline]
 cont_write_begin+0x18d/0x920 fs/buffer.c:2586
 fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
 generic_cont_expand_simple+0xb0/0x150 fs/buffer.c:2487
 fat_cont_expand+0x3e/0x170 fs/fat/file.c:227
 fat_setattr+0x2a4/0x850 fs/fat/file.c:551
 notify_change+0x842/0x8c0 fs/attr.c:556
 do_truncate fs/open.c:68 [inline]
 do_ftruncate+0x344/0x410 fs/open.c:194
 ksys_ftruncate fs/open.c:206 [inline]
 __do_sys_ftruncate fs/open.c:211 [inline]
 __se_sys_ftruncate fs/open.c:209 [inline]
 __x64_sys_ftruncate+0x65/0xc0 fs/open.c:209
 x64_sys_call+0x2beb/0x3020 arch/x86/include/generated/asm/syscalls_64.h:78
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811aa250f0 of 8 bytes by task 26371 on cpu 1:
 mapping_needs_writeback mm/filemap.c:635 [inline]
 filemap_write_and_wait_range+0x59/0x370 mm/filemap.c:684
 kiocb_write_and_wait mm/filemap.c:2905 [inline]
 generic_file_read_iter+0xf4/0x320 mm/filemap.c:2979
 copy_splice_read+0x47a/0x6b0 fs/splice.c:362
 do_splice_read fs/splice.c:979 [inline]
 splice_direct_to_actor+0x27c/0x680 fs/splice.c:1084
 do_splice_direct_actor fs/splice.c:1202 [inline]
 do_splice_direct+0x119/0x1a0 fs/splice.c:1228
 do_sendfile+0x37d/0x640 fs/read_write.c:1371
 __do_sys_sendfile64 fs/read_write.c:1432 [inline]
 __se_sys_sendfile64 fs/read_write.c:1418 [inline]
 __x64_sys_sendfile64+0x105/0x150 fs/read_write.c:1418
 x64_sys_call+0x2dc4/0x3020 arch/x86/include/generated/asm/syscalls_64.h:41
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x000000000000002f -> 0x0000000000000030

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 26371 Comm: syz.8.9978 Tainted: G        W           syzkaller #0 PREEMPT(lazy) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
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