Re: [PATCH v5] mm/page_alloc: only update lowmem_reserve_ratio on sysctl write

[Andrew Morton]

On Thu, 6 Aug 2026 09:50:16 -0400 Johannes Weiner <hannes@xxxxxxxxxxx> wrote:

> > -	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;
> > -	}
> 
> This could use a comment. How about:
> 
> 	/*
> 	 * proc_dointvec_max() works incrementally. Use a buffer
> 	 * and only set the values if all of them parse cleanly.
> 	 */
> 

I added a fixlet:

--- a/mm/page_alloc.c~mm-page_alloc-only-update-lowmem_reserve_ratio-on-sysctl-write-fix
+++ a/mm/page_alloc.c
@@ -6932,6 +6932,10 @@ static int lowmem_reserve_ratio_sysctl_h
 	if (!write)
 		return proc_dointvec_minmax(table, write, buffer, length, ppos);
 
+	/*
+	 * proc_dointvec_max() works incrementally. Use a buffer and only set
+	 * the values if all of them parse cleanly.
+	 */
 	memcpy(ratio, sysctl_lowmem_reserve_ratio, sizeof(ratio));
 	tmp.data = ratio;
 
_

Sashiko got all upset about concurrency issues:

	https://sashiko.dev/#/patchset/tencent_A860C873956A52E26AD8D309A308A241BA08@xxxxxx

But I think I'm hearing from Joel is that the longer-term plan is to fix
all this up within proc_dointvec_minmax()?