linux-kernel - Re: [PATCH 4/6] mm/mm_init.c: remove meaningless calculation of zone->managed_pages in free_area_init

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Date: Wed, 20 Mar 2024 16:18:18 +0800
From: Baoquan He <bhe@...hat.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, x86@...nel.org,
	linuxppc-dev@...ts.ozlabs.org, akpm@...ux-foundation.org
Subject: Re: [PATCH 4/6] mm/mm_init.c: remove meaningless calculation of
 zone->managed_pages in free_area_init_core()

On 03/19/24 at 06:17pm, Mike Rapoport wrote:
> On Mon, Mar 18, 2024 at 10:21:36PM +0800, Baoquan He wrote:
> > Currently, in free_area_init_core(), when initialize zone's field, a
> > rough value is set to zone->managed_pages. That value is calculated by
> > (zone->present_pages - memmap_pages).
> > 
> > In the meantime, add the value to nr_all_pages and nr_kernel_pages which
> > represent all free pages of system (only low memory or including HIGHMEM
> > memory separately). Both of them are gonna be used in
> > alloc_large_system_hash().
> > 
> > However, the rough calculation and setting of zone->managed_pages is
> > meaningless because
> >   a) memmap pages are allocated on units of node in sparse_init() or
> >      alloc_node_mem_map(pgdat); The simple (zone->present_pages -
> >      memmap_pages) is too rough to make sense for zone;
> >   b) the set zone->managed_pages will be zeroed out and reset with
> >      acutal value in mem_init() via memblock_free_all(). Before the
> >      resetting, no buddy allocation request is issued.
> > 
> > Here, remove the meaningless and complicated calculation of
> > (zone->present_pages - memmap_pages), directly set zone->present_pages to
> > zone->managed_pages. It will be adjusted in mem_init().
> 
> Do you mean "set zone->managed_pages to zone->present_pages"?

Hmm, maybe 'set zone->managed_pages as zone->present_pages'
            or 
           'assign zone->present_pages to zone->managed_pages'
which is more precise.

Wwill update.

> 
> I think we can just set zone->managed_pages to 0 in free_area_init_core().
> Anyway it will be reset before the first use.

Yeah, setting to 0 is also fine. I thougt of 0 ever. Considering
zone->present_pages is closer value to actual zone->managed_pages
than 0, and it may be needed in the future in some way before
mem_init(). If no strong objection, I will keep the assigning
'zone->present_pages' to 'zone->managed_pages'.

Thanks again for careful reviewing.