mm/page_alloc: clarify batch tuning in zone_batchsize

mm/page_alloc: clarify batch tuning in zone_batchsize

Patch series "mm/page_alloc: pcp->batch cleanups", v2.

Two small cleanups for mm/page_alloc.

Patch 1 cleans up a misleading comment about how pcp->batch is calculated,
and folds in the calculation to increase clarity. No functional change
intended.

Patch 2 corrects zones from reporting that their pcp->batch is 0 when it
is actually 1. Namely, corrects ZONE_DMA from reporting that its batch
size is 0.

This patch (of 2):

Recently while working on another patch about batching free_pcppages_bulk
[1], I was curious why pcp->batch was always 63 on my machine.  This led
me to zone_batchsize(), where I found this set of lines to determine what
the batch size should be for the host:

	batch = min(zone_managed_pages(zone) >> 10, SZ_1M / PAGE_SIZE);
	batch /= 4;		/* We effectively *= 4 below */
	if (batch < 1)
		batch = 1;

All of this is good, except the comment above which says "We effectively
*= 4 below".  Nowhere else in the function zone_batchsize(), is there a
corresponding multipliation by 4.  Looking into the history of this, it
seems like Dave Hansen had also noticed this back in 2013 [1].  Turns out
there *used* to be a corresponding *= 4, which was turned into a *= 6
later on to be used in pageset_setup_from_batch_size(), which no longer
exists.

Despite this mismatch not being corrected in the comments, it seems that
getting rid of the /= 4 leads to a performance regression on machines with
less than 250G memory and 176 processors.  As such, let us preserve the
functionality but clean up the comments.

Fold the /= 4 into the calculation above: bitshift by 10+2=12, and instead
of dividing 1MB, divide 256KB and adjust the comments accordingly.  No
functional change intended.

Link: https://lkml.kernel.org/r/20251009192933.3756712-1-joshua.hahnjy@gmail.com
Link: https://lkml.kernel.org/r/20251009192933.3756712-2-joshua.hahnjy@gmail.com
Link: https://lore.kernel.org/all/20251002204636.4016712-1-joshua.hahnjy@gmail.com/
Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Suggested-by: Dave Hansen <dave.hansen@intel.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
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: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>