mm/page_alloc: only update NUMA min ratios on sysctl write

mm/page_alloc: only update NUMA min ratios on sysctl write

The sysctl handlers for min_unmapped_ratio and min_slab_ratio invoke
setup_min_unmapped_ratio() and setup_min_slab_ratio() unconditionally
after proc_dointvec_minmax(), even for read operations.

These setup functions first zero all per-NUMA node thresholds
(min_unmapped_pages and min_slab_pages) before recalculating them.
Reading /proc sysctl entries therefore temporarily resets node reclaim
thresholds to zero, which may disturb the behavior of __node_reclaim() and
node_reclaim() during the recomputation.

Fix this by only calling the setup functions when the sysctl is actually
written (write == 1), matching the behavior of existing sysctl handlers
like min_free_kbytes and watermark_scale_factor.

This only affects systems with CONFIG_NUMA.

Link: https://lore.kernel.org/tencent_5891052AF9A4C2D490A62F478D446F74AB09@qq.com
Signed-off-by: Jianlin Shi <shijianlin11@foxmail.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>