[PATCH v3] mm/page_alloc: only update lowmem_reserve_ratio on sysctl write

lowmem_reserve_ratio_sysctl_handler() ignores the return value of
proc_dointvec_minmax() and always calls setup_per_zone_lowmem_reserve(),
even for read operations.

Fix two issues:

1. Propagate errors from proc_dointvec_minmax() instead of always
   returning success.  For example, writing non-integer garbage to the
   sysctl now returns an error instead of silently succeeding with
   unchanged values.

2. Only call setup_per_zone_lowmem_reserve() when the sysctl is
   actually written.  Reading /proc/sys/vm/lowmem_reserve_ratio should
   not recompute derived lowmem_reserve[] and totalreserve_pages when
   the inputs did not change, matching min_free_kbytes and
   watermark_scale_factor handlers.

Drop the manual "< 1 -> 0" sanitization loop and set .extra1 =
SYSCTL_ZERO on the ctl_table entry so proc_dointvec_minmax() enforces
the minimum on write; negative values now return -EINVAL instead of
being silently coerced to 0 (suggested by Vlastimil Babka).

Link: https://lore.kernel.org/linux-mm/[email protected]/

Reviewed-by: Vlastimil Babka (SUSE) <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Signed-off-by: Jianlin Shi <[email protected]>
---
Changes in v3:
- Rewrite commit log to focus on the two tangible fixes as suggested by
  Johannes Weiner.
- Code remains unchanged versus v2; retain previously obtained tags.

Changes in v2:
- Add .extra1 = SYSCTL_ZERO to ctl_table entry
- Remove manual sanitization loop; negative writes now return -EINVAL

v1: https://lore.kernel.org/linux-mm/[email protected]/

 mm/page_alloc.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 0387d2afd..a7381327d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6683,16 +6683,15 @@ static int sysctl_min_slab_ratio_sysctl_handler(const struct ctl_table *table, i
 static int lowmem_reserve_ratio_sysctl_handler(const struct ctl_table *table,
 		int write, void *buffer, size_t *length, loff_t *ppos)
 {
-	int i;
+	int rc;
 
-	proc_dointvec_minmax(table, write, buffer, length, ppos);
+	rc = proc_dointvec_minmax(table, write, buffer, length, ppos);
+	if (rc)
+		return rc;
 
-	for (i = 0; i < MAX_NR_ZONES; i++) {
-		if (sysctl_lowmem_reserve_ratio[i] < 1)
-			sysctl_lowmem_reserve_ratio[i] = 0;
-	}
+	if (write)
+		setup_per_zone_lowmem_reserve();
 
-	setup_per_zone_lowmem_reserve();
 	return 0;
 }
 
@@ -6791,6 +6790,7 @@ static const struct ctl_table page_alloc_sysctl_table[] = {
 		.maxlen		= sizeof(sysctl_lowmem_reserve_ratio),
 		.mode		= 0644,
 		.proc_handler	= lowmem_reserve_ratio_sysctl_handler,
+		.extra1		= SYSCTL_ZERO,
 	},
 #ifdef CONFIG_NUMA
 	{
-- 
2.43.0