[PATCH v4] 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 three 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, matching the write-only refresh pattern of
   min_free_kbytes and watermark_scale_factor handlers.

3. On write, parse into a temporary ratio[] array and only copy into
   sysctl_lowmem_reserve_ratio[] and refresh derived state after the
   full vector is validated.  This avoids leaving the ratio array
   partially updated while skipping setup when proc_dointvec_minmax()
   returns an error on a later element (suggested by Andrew Morton).

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]/

Signed-off-by: Jianlin Shi <[email protected]>
---
Changes in v4:
- Parse writes into a temporary ratio[] array; commit and setup only on
  full success (Andrew Morton)
- Update handler comment: proc_dointvec_minmax() and write-only setup

Changes in v3:
- Rewrite commit log to focus on the two tangible fixes as suggested by
  Johannes Weiner.

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 | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 0387d2afd..13fabb062 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6673,8 +6673,8 @@ static int sysctl_min_slab_ratio_sysctl_handler(const struct ctl_table *table, i
 
 /*
  * lowmem_reserve_ratio_sysctl_handler - just a wrapper around
- *	proc_dointvec() so that we can call setup_per_zone_lowmem_reserve()
- *	whenever sysctl_lowmem_reserve_ratio changes.
+ *	proc_dointvec_minmax() so that we can call
+ *	setup_per_zone_lowmem_reserve() when the sysctl is written.
  *
  * The reserve ratio obviously has absolutely no relation with the
  * minimum watermarks. The lowmem reserve ratio can only make sense
@@ -6683,16 +6683,23 @@ 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;
+	struct ctl_table tmp = *table;
+	int ratio[MAX_NR_ZONES];
+	int rc;
 
-	proc_dointvec_minmax(table, write, buffer, length, ppos);
+	if (!write)
+		return proc_dointvec_minmax(table, write, buffer, length, ppos);
 
-	for (i = 0; i < MAX_NR_ZONES; i++) {
-		if (sysctl_lowmem_reserve_ratio[i] < 1)
-			sysctl_lowmem_reserve_ratio[i] = 0;
-	}
+	memcpy(ratio, sysctl_lowmem_reserve_ratio, sizeof(ratio));
+	tmp.data = ratio;
 
+	rc = proc_dointvec_minmax(&tmp, write, buffer, length, ppos);
+	if (rc)
+		return rc;
+
+	memcpy(sysctl_lowmem_reserve_ratio, ratio, sizeof(ratio));
 	setup_per_zone_lowmem_reserve();
+
 	return 0;
 }
 
@@ -6791,6 +6798,7 @@ 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