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

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Date:   Tue, 4 Feb 2020 09:40:30 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Alexander Duyck <alexander.duyck@...il.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 04.02.20 00:17, Alexander Duyck wrote:
> On Mon, Feb 3, 2020 at 1:44 PM David Hildenbrand <david@...hat.com> wrote:
>>
>>
>>
>>> 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?
> 
> Actually I hadn't read it all that closely, so my bad on that. The
> part that had caught my attention though was that
> memblock_region_memory_end is using PFN_DOWN to identify the end of
> the memory region, Given that we probably shouldn't be messing with
> the PFNs that may contain any of this memory it might make more sense
> to use memblock_region_reserved_end_pfn which uses PFN_UP so that we
> exclude all memory that is in the mirrored region just in case
> something doesn't end on a PFN aligned boundary.
> 
> If we know that the mirrored region is going to always be page size
> aligned then I guess you are good to go. That was the only thing I
> wasn't sure about.

I think we can safely assume this for now. But I *think* we are fine
either way:

We are using memblock_region_memory_end() in all cases I spotted
(especially consistently in overlap_memmap_init()) - so there is never a
mis-match that could result in an endless loop.

Anyhow, having mirrored sub-page regions would be weird either way :)
(just like any zone that would end on sub-pages)

> 
> Reviewed-by: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
> 

Thanks!

-- 
Thanks,

David / dhildenb