linux-kernel - Re: [PATCH v1 1/2] mm/page_alloc: fix and rework pfn handling in memmap_init

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Message-ID: <CAKgT0UdFphMbS04NMRYU=VUmbnVi_purz4UA0cqA3dd7YW-pJA@mail.gmail.com>
Date:   Mon, 3 Feb 2020 13:35:29 -0800
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Oscar Salvador <osalvador@...e.de>,
        "Kirill A . Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH v1 1/2] mm/page_alloc: fix and rework pfn handling in memmap_init_zone()

On Mon, Jan 13, 2020 at 6:40 AM David Hildenbrand <david@...hat.com> wrote:
>
> Let's update the pfn manually whenever we continue the loop. This makes
> the code easier to read but also less error prone (and we can directly
> fix one issue).
>
> When overlap_memmap_init() returns true, pfn is updated to
> "memblock_region_memory_end_pfn(r)". So it already points at the *next*
> pfn to process. Incrementing the pfn another time is wrong, we might
> leave one uninitialized. I spotted this by inspecting the code, so I have
> no idea if this is relevant in practise (with kernelcore=mirror).
>
> Fixes: a9a9e77fbf27 ("mm: move mirrored memory specific code outside of memmap_init_zone")
> Cc: Pavel Tatashin <pasha.tatashin@...cle.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Kirill A. Shutemov <kirill@...temov.name>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  mm/page_alloc.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index a41bd7341de1..a92791512077 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5905,18 +5905,20 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>         }
>  #endif
>
> -       for (pfn = start_pfn; pfn < end_pfn; pfn++) {
> +       for (pfn = start_pfn; pfn < end_pfn; ) {
>                 /*
>                  * There can be holes in boot-time mem_map[]s handed to this
>                  * function.  They do not exist on hotplugged memory.
>                  */
>                 if (context == MEMMAP_EARLY) {
>                         if (!early_pfn_valid(pfn)) {
> -                               pfn = next_pfn(pfn) - 1;
> +                               pfn = next_pfn(pfn);
>                                 continue;
>                         }
> -                       if (!early_pfn_in_nid(pfn, nid))
> +                       if (!early_pfn_in_nid(pfn, nid)) {
> +                               pfn++;
>                                 continue;
> +                       }
>                         if (overlap_memmap_init(zone, &pfn))
>                                 continue;
>                         if (defer_init(nid, pfn, end_pfn))

I'm pretty sure this is a bit broken. The overlap_memmap_init is going
to return memblock_region_memory_end_pfn instead of the start of the
next region. I think that is going to stick you in a mirrored region
without advancing in that case. You would also need to have that case
do a pfn++ before the continue;

> @@ -5944,6 +5946,7 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>                         set_pageblock_migratetype(page, MIGRATE_MOVABLE);
>                         cond_resched();
>                 }
> +               pfn++;
>         }
>  }
>
> --
> 2.24.1
>
>