Re: [PATCH] mm/page_alloc: restore 0-handling to zone_set_pageset_high_and_batch

[Vlastimil Babka <vbabka@suse.cz>]

On 12/17/25 12:12, Vlastimil Babka wrote:
> On 12/17/25 07:05, Joshua Hahn wrote:
>> Commit 2783088ef24e ("mm/page_alloc: prevent reporting pcp->batch = 0")
>> moved the error handling (0-handling) of zone_batchsize from its
>> callers to inside the function. However, the commit left out the error
>> handling for the NOMMU case, leading to deadlocks on NOMMU systems.
>> 
>> Since in the NOMMU case the reported-to-user batchsize should still be 0,
> 
> Should it? The value is effectively set to 1 despite what zone_batchsize()
> returns, because of that adjustment this patch reinstates. Also does anyone
> care, really?
> 
>> we would only like the error handling to exist in the callsites that
>> set the internal value for the zone (i.e. zone_set_pageset_high_and_batch).
>> 
>> Restore max(1, zone_batchsize(zone)) to the callsite to prevent errors
>> on NOMMU systems.
> 
> I would rather make zone_batchsize() for !CONFIG_MMU return 1 instead of 0.

Ah looks like you considered it too, initially:
https://lore.kernel.org/all/20251211225947.822866-1-joshua.hahnjy@gmail.com/

It makes more sense to me than doing effectively two fixups in the MMU case.

>> Fixes: 2783088ef24e ("mm/page_alloc: prevent reporting pcp->batch = 0")
>> Reported-by: Daniel Palmer <daniel@thingy.jp>
>> Closes: https://lore.kernel.org/linux-mm/CAFr9PX=_HaM3_xPtTiBn5Gw5-0xcRpawpJ02NStfdr0khF2k7g@mail.gmail.com/
>> Reported-by: Guenter Roeck <linux@roeck-us.net>
>> Closes: https://lore.kernel.org/all/42143500-c380-41fe-815c-696c17241506@roeck-us.net/
>> Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
>> ---
>>  mm/page_alloc.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 822e05f1a964..10c1297fd3ea 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -6045,7 +6045,7 @@ static void zone_set_pageset_high_and_batch(struct zone *zone, int cpu_online)
>>  {
>>  	int new_high_min, new_high_max, new_batch;
>>  
>> -	new_batch = zone_batchsize(zone);
>> +	new_batch = max(1, zone_batchsize(zone));
>>  	if (percpu_pagelist_high_fraction) {
>>  		new_high_min = zone_highsize(zone, new_batch, cpu_online,
>>  					     percpu_pagelist_high_fraction);
>> 
>> base-commit: 40fbbd64bba6c6e7a72885d2f59b6a3be9991eeb
>