linux-kernel - Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region when MEMINIT

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Message-ID: <8c9ee3bd-6d71-4111-8f4e-91bc52b42ed4@linux.dev>
Date:   Tue, 3 Oct 2023 22:38:09 +0800
From:   Yajun Deng <yajun.deng@...ux.dev>
To:     David Hildenbrand <david@...hat.com>,
        Mike Rapoport <rppt@...nel.org>
Cc:     akpm@...ux-foundation.org, mike.kravetz@...cle.com,
        muchun.song@...ux.dev, willy@...radead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region when
 MEMINIT_EARLY


On 2023/10/2 19:25, David Hildenbrand wrote:
> On 02.10.23 13:10, Mike Rapoport wrote:
>> On Mon, Oct 02, 2023 at 10:56:51AM +0200, David Hildenbrand wrote:
>>> On 02.10.23 10:47, Mike Rapoport wrote:
>>>> On Mon, Oct 02, 2023 at 03:03:56PM +0800, Yajun Deng wrote:
>>>>>
>>>>> On 2023/10/2 02:59, Mike Rapoport wrote:
>>>>>> On Fri, Sep 29, 2023 at 06:27:25PM +0800, Yajun Deng wrote:
>>>>>>> On 2023/9/29 18:02, Mike Rapoport wrote:
>>>>>>>>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>>>>>>>>> index 06be8821d833..b868caabe8dc 100644
>>>>>>>>>>> --- a/mm/page_alloc.c
>>>>>>>>>>> +++ b/mm/page_alloc.c
>>>>>>>>>>> @@ -1285,18 +1285,22 @@ void __free_pages_core(struct page 
>>>>>>>>>>> *page, unsigned int order)
>>>>>>>>>>>           unsigned int loop;
>>>>>>>>>>>           /*
>>>>>>>>>>> -     * When initializing the memmap, __init_single_page() 
>>>>>>>>>>> sets the refcount
>>>>>>>>>>> -     * of all pages to 1 ("allocated"/"not free"). We have 
>>>>>>>>>>> to set the
>>>>>>>>>>> -     * refcount of all involved pages to 0.
>>>>>>>>>>> +     * When initializing the memmap, memmap_init_range sets 
>>>>>>>>>>> the refcount
>>>>>>>>>>> +     * of all pages to 1 ("reserved" and "free") in hotplug 
>>>>>>>>>>> context. We
>>>>>>>>>>> +     * have to set the refcount of all involved pages to 0. 
>>>>>>>>>>> Otherwise,
>>>>>>>>>>> +     * we don't do it, as reserve_bootmem_region only set 
>>>>>>>>>>> the refcount on
>>>>>>>>>>> +     * reserve region ("reserved") in early context.
>>>>>>>>>>>            */
>>>>>>>>>> Again, why hotplug and early init should be different?
>>>>>>>>> I will add a comment that describes it will save boot time.
>>>>>>>> But why do we need initialize struct pages differently at boot 
>>>>>>>> time vs
>>>>>>>> memory hotplug?
>>>>>>>> Is there a reason memory hotplug cannot have page count set to 
>>>>>>>> 0 just like
>>>>>>>> for pages reserved at boot time?
>>>>>>> This patch just save boot time in MEMINIT_EARLY. If someone 
>>>>>>> finds out that
>>>>>>> it can save time in
>>>>>>>
>>>>>>> MEMINIT_HOTPLUG, I think it can be done in another patch later. 
>>>>>>> I just
>>>>>>> keeping it in the same.
>>>>>> But it's not the same. It becomes slower after your patch and the 
>>>>>> code that
>>>>>> frees the pages for MEMINIT_EARLY and MEMINIT_HOTPLUG becomes 
>>>>>> non-uniform
>>>>>> for no apparent reason.
>>>>>
>>>>> __free_pages_core will also be called by others, such as:
>>>>> deferred_free_range, do_collection and memblock_free_late.
>>>>>
>>>>> We couldn't remove  'if (page_count(page))' even if we set page 
>>>>> count to 0
>>>>> when MEMINIT_HOTPLUG.
>>>>
>>>> That 'if' breaks the invariant that __free_pages_core is always 
>>>> called for
>>>> pages with initialized page count. Adding it may lead to subtle 
>>>> bugs and
>>>> random memory corruption so we don't want to add it at the first 
>>>> place.
>>>
>>> As long as we have to special-case memory hotplug, we know that we are
>>> always coming via generic_online_page() in that case. We could 
>>> either move
>>> some logic over there, or let __free_pages_core() know what it 
>>> should do.
>>
>> Looks like the patch rather special cases MEMINIT_EARLY, although I 
>> didn't
>> check throughfully other code paths.
>> Anyway, relying on page_count() to be correct in different ways for
>> different callers of __free_pages_core() does not sound right to me.
>
> Absolutely agreed.
>
I already sent v5  a few days ago. Comments, please...