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. Reading /proc/sys/vm/lowmem_reserve_ratio should not recompute per-zone lowmem_reserve[] and totalreserve_pages. Only do so when the sysctl is written, matching min_free_kbytes and watermark_scale_factor handlers. Also propagate errors from proc_dointvec_minmax() instead of ignoring them. Drop the manual "< 1 -> 0" sanitization loop in the handler and set .extra1 = SYSCTL_ZERO on the ctl_table entry so proc_dointvec_minmax() enforces the minimum on write (suggested by Vlastimil Babka). Compatibility note: Previously a read also sanitized sysctl_lowmem_reserve_ratio[] and called setup_per_zone_lowmem_reserve(), which rewrites each zone's lowmem_reserve[] and recalculates pgdat->totalreserve_pages / totalreserve_pages (visible via /proc/zoneinfo "protection" and used by page allocation fallback and dirty-limit accounting). After this change only a write does that. Documentation describes the meaning of the ratio and the derived protection pages, but does not document any read side-effect. Worst case for odd userspace that treated a read as a refresh of those derived values: lowmem_reserve[] and totalreserve_pages remain at their last written/setup values until the next write of this sysctl, or until another existing updater runs (e.g. adjust_managed_page_count() on managed-page changes, or init/watermark setup paths). Until then, allocation fallback into lower zones and per-node dirtyable memory (node_dirtyable_memory() subtracts pgdat->totalreserve_pages) may not reflect a refresh that such userspace expected from the read alone. Normal readers that only consume the ratio array are unaffected. Changes in v2: - Add .extra1 = SYSCTL_ZERO to the ctl_table entry - Remove the manual sanitization loop (negative writes now return -EINVAL instead of being silently coerced to 0) Link: https://lore.kernel.org/linux-mm/tencent_FFD4F4D728AAE8A8AE0AF277A59854A29A06@qq.com/ Signed-off-by: Jianlin Shi --- 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