Re: [PATCH v4] mm/page_alloc: prevent reporting pcp->batch = 0

[Andrew Morton]

On Tue, 16 Dec 2025 06:48:11 -0800 Joshua Hahn <joshua.hahnjy@xxxxxxxxx> wrote:

> zone_batchsize returns the appropriate value that should be used for
> pcp->batch. If it finds a zone with less than 4096 pages or PAGE_SIZE >
> 1M, however, it leads to some incorrect math.
> 
> In the case above, we will get an intermediary value of 1, which is then
> rounded down to the nearest power of two, and 1 is subtracted from it.
> Since 1 is already a power of two, we will get batch = 1-1 = 0:
> 
>         batch = rounddown_pow_of_two(batch + batch/2) - 1;
> 
> A pcp->batch value of 0 is nonsensical, for MMU systems. If this were
> actually set, then functions like drain_zone_pages would become no-ops,
> since they would free 0 pages at a time.
> 
> Of the two callers of zone_batchsize, the one that is actually used to
> set pcp->batch works around this by setting pcp->batch to the maximum
> of 1 and zone_batchsize. However, the other caller, zone_pcp_init,
> incorrectly prints out the batch size of the zone to be 0.
> 
> This is probably rare in a typical zone, but the DMA zone can often have
> less than 4096 pages, which means it will print out "LIFO batch:0".
> 
> Before: [    0.001216]   DMA zone: 3998 pages, LIFO batch:0
> After:  [    0.001210]   DMA zone: 3998 pages, LIFO batch:1
> 
> With all of this said, NOMMU differs in two ways. Semantically, it
> should report that pcp->batch is 0. At the same time, it can never
> really have a pcp->batch size of 0 since it will reach a deadlock in
> pcp freeing functions. For this reason, zone_batchsize should still
> report 0 for NOMMU, but zone_set_pageset_high_and_batch should still
> interpret it as 1, meaning we cannot get rid of max(1, zone_batchsize())
> in zone_set_pageset_high_and_batch.
> 
> Suggested-by: Daniel Palmer <daniel@xxxxxxxx>
> Signed-off-by: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
> ---
> Reviewers' note:
> 
> This patch was originally a part of the 6.19-rc1 pr, but Daniel Palmer
> kindly reported that this patch causes an issue on NOMMU systems [1].
> Thank you, Daniel! I wasn't sure how to credit here since it was a
> report on an unmerged commit so I went with suggested-by. If this is
> problematic please let me know and I will change the tag.
> 
> [1] https://lore.kernel.org/all/CAFr9PX=_HaM3_xPtTiBn5Gw5-0xcRpawpJ02NStfdr0khF2k7g@xxxxxxxxxxxxxx/
> 
> Reviewer's note (to Andrew):
> 
> This replaces commit 2/2 of the series titled "mm/page_alloc: pcp->batch
> cleanups" [2].

That series is in mainline.  2783088ef24e ("mm/page_alloc: prevent
reporting pcp->batch = 0").

> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5888,8 +5888,8 @@ static int zone_batchsize(struct zone *zone)
>  	 * and zone lock contention.
>  	 */
>  	batch = min(zone_managed_pages(zone) >> 12, SZ_256K / PAGE_SIZE);
> -	if (batch < 1)
> -		batch = 1;
> +	if (batch <= 1)
> +		return 1;
>  
>  	/*
>  	 * Clamp the batch to a 2^n - 1 value. Having a power
> 

So this doesn't work.  Please send along a fix for current -linus and include

Fixes: 2783088ef24e ("mm/page_alloc: prevent reporting pcp->batch = 0")

and Reported-by:Daniel and the appropriate Closes:

Thanks.