x86: remove unneeded memblock_find_dma_reserve()

x86: remove unneeded memblock_find_dma_reserve()

Patch series "mm/mm_init.c: refactor free_area_init_core()".

In function free_area_init_core(), the code calculating
zone->managed_pages and the subtracting dma_reserve from DMA zone looks
very confusing.

From git history, the code calculating zone->managed_pages was for
zone->present_pages originally.  The early rough assignment is for
optimize zone's pcp and water mark setting.  Later, managed_pages was
introduced into zone to represent the number of managed pages by buddy.
Now, zone->managed_pages is zeroed out and reset in mem_init() when
calling memblock_free_all().  zone's pcp and wmark setting relying on
actual zone->managed_pages are done later than mem_init() invocation.  So
we don't need rush to early calculate and set zone->managed_pages, just
set it as zone->present_pages, will adjust it in mem_init().

And also add a new function calc_nr_kernel_pages() to count up free but
not reserved pages in memblock, then assign it to nr_all_pages and
nr_kernel_pages after memmap pages are allocated.

This patch (of 6):

Variable dma_reserve and its usage was introduced in commit 0e0b864e069c
("[PATCH] Account for memmap and optionally the kernel image as holes").
Its original purpose was to accounting for the reserved pages in DMA zone
to make DMA zone's watermarks calculation more accurate on x86.

However, currently there's zone->managed_pages to account for all
available pages for buddy, zone->present_pages to account for all present
physical pages in zone.  What is more important, on x86, calculating and
setting the zone->managed_pages is a temporary move, all zone's
managed_pages will be zeroed out and reset to the actual value according
to how many pages are added to buddy allocator in mem_init().  Before
mem_init(), no buddy alloction is requested.  And zone's pcp and watermark
setting are all done after mem_init().  So, no need to worry about the DMA
zone's setting accuracy during free_area_init().

Hence, remove memblock_find_dma_reserve() to stop calculating and
setting dma_reserve.

Link: https://lkml.kernel.org/r/20240325145646.1044760-1-bhe@redhat.com
Link: https://lkml.kernel.org/r/20240325145646.1044760-2-bhe@redhat.com
Signed-off-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>