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

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Message-Id: <1583F4CF-6CD8-4AB6-A2F6-60E6AEE5D5B2@redhat.com>
Date:   Mon, 3 Feb 2020 22:44:45 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     David Hildenbrand <david@...hat.com>,
        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()



> Am 03.02.2020 um 22:35 schrieb Alexander Duyck <alexander.duyck@...il.com>:
> 
> 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;

Thanks for having a look.

Did you read the description regarding this change?