Re: [PATCH v2 2/2] mm/page_alloc: Prevent reporting pcp->batch = 0

[Vlastimil Babka <vbabka@suse.cz>]

On 10/9/25 21:29, Joshua Hahn 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 above case, 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. If this were actually set, then
> functions like drain_zone_pages would become no-ops, since they could
> only 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
> 
> Instead of dealing with the error handling and the mismatch between the
> reported and actual zone batchsize, just return 1 if the zone_batchsize
> is 1 page or less before the rounding.
> 
> Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>