[PATCH 4/4] s390: ptdump: use seq_puts() in pt_dump_seq_puts() macro

[PATCH 4/4] s390: ptdump: use seq_puts() in pt_dump_seq_puts() macro

[Josephine Pfeiffer]

The pt_dump_seq_puts() macro incorrectly uses seq_printf() instead of
seq_puts(). This is both a performance issue and conceptually wrong,
as the macro name suggests plain string output (puts) but the
implementation uses formatted output (printf).

The macro is used in dump_pagetables.c:67-68 and 131 to output
constant strings. Using seq_printf() adds unnecessary overhead for
format string parsing.

This bug was introduced in commit 6bf9a639e76e1 ("s390/mm,ptdump: make
page table dumping seq_file optional") in 2020, which explicitly stated
it was the "s390 version of commit ae5d1cf358a5 ("arm64: dump: Make the
page table dumping seq_file optional")", copying the buggy arm64
implementation.

Fixes: 6bf9a639e76e1 ("s390/mm,ptdump: make page table dumping seq_file optional")
Signed-off-by: Josephine Pfeiffer <hi@josie.lol>
---
 arch/s390/mm/dump_pagetables.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/s390/mm/dump_pagetables.c b/arch/s390/mm/dump_pagetables.c
index 9af2aae0a515..3692f9d20f0d 100644
--- a/arch/s390/mm/dump_pagetables.c
+++ b/arch/s390/mm/dump_pagetables.c
@@ -51,7 +51,7 @@ struct pg_state {
 	struct seq_file *__m = (m);		\
 						\
 	if (__m)				\
-		seq_printf(__m, fmt);		\
+		seq_puts(__m, fmt);		\
 })
 
 static void print_prot(struct seq_file *m, unsigned int pr, int level)
-- 
2.51.1.dirty

Re: [PATCH 4/4] s390: ptdump: use seq_puts() in pt_dump_seq_puts() macro

[Heiko Carstens]

On Sat, Oct 18, 2025 at 07:05:21PM +0200, Josephine Pfeiffer wrote:
> The pt_dump_seq_puts() macro incorrectly uses seq_printf() instead of
> seq_puts(). This is both a performance issue and conceptually wrong,
> as the macro name suggests plain string output (puts) but the
> implementation uses formatted output (printf).
> 
> The macro is used in dump_pagetables.c:67-68 and 131 to output
> constant strings. Using seq_printf() adds unnecessary overhead for
> format string parsing.
> 
> This bug was introduced in commit 6bf9a639e76e1 ("s390/mm,ptdump: make
> page table dumping seq_file optional") in 2020, which explicitly stated
> it was the "s390 version of commit ae5d1cf358a5 ("arm64: dump: Make the
> page table dumping seq_file optional")", copying the buggy arm64
> implementation.
> 
> Fixes: 6bf9a639e76e1 ("s390/mm,ptdump: make page table dumping seq_file optional")
> Signed-off-by: Josephine Pfeiffer <hi@josie.lol>
...
> -		seq_printf(__m, fmt);		\
> +		seq_puts(__m, fmt);		\

So yes, it should have been seq_puts() instead of seq_printf(), but
there really is no _bug_ here, except of wasted CPU cycles for a seq
file which isn't enabled in any production kernel.

I'll apply this and change the commit message accordingly.

Thanks!