Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region when MEMINIT_EARLY

[Yajun Deng <yajun.deng@linux.dev>]

On 2023/10/8 16:57, Yajun Deng wrote:
>
> On 2023/10/2 16:30, David Hildenbrand wrote:
>> On 29.09.23 10:30, Mike Rapoport wrote:
>>> On Thu, Sep 28, 2023 at 04:33:02PM +0800, Yajun Deng wrote:
>>>> memmap_init_range() would init page count of all pages, but the free
>>>> pages count would be reset in __free_pages_core(). There are opposite
>>>> operations. It's unnecessary and time-consuming when it's 
>>>> MEMINIT_EARLY
>>>> context.
>>>>
>>>> Init page count in reserve_bootmem_region when in MEMINIT_EARLY 
>>>> context,
>>>> and check the page count before reset it.
>>>>
>>>> At the same time, the INIT_LIST_HEAD in reserve_bootmem_region isn't
>>>> need, as it already done in __init_single_page.
>>>>
>>>> The following data was tested on an x86 machine with 190GB of RAM.
>>>>
>>>> before:
>>>> free_low_memory_core_early()    341ms
>>>>
>>>> after:
>>>> free_low_memory_core_early()    285ms
>>>>
>>>> Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
>>>> ---
>>>> v4: same with v2.
>>>> v3: same with v2.
>>>> v2: check page count instead of check context before reset it.
>>>> v1: 
>>>> https://lore.kernel.org/all/20230922070923.355656-1-yajun.deng@linux.dev/
>>>> ---
>>>>   mm/mm_init.c    | 18 +++++++++++++-----
>>>>   mm/page_alloc.c | 20 ++++++++++++--------
>>>>   2 files changed, 25 insertions(+), 13 deletions(-)
>>>>
>>>> diff --git a/mm/mm_init.c b/mm/mm_init.c
>>>> index 9716c8a7ade9..3ab8861e1ef3 100644
>>>> --- a/mm/mm_init.c
>>>> +++ b/mm/mm_init.c
>>>> @@ -718,7 +718,7 @@ static void __meminit 
>>>> init_reserved_page(unsigned long pfn, int nid)
>>>>           if (zone_spans_pfn(zone, pfn))
>>>>               break;
>>>>       }
>>>> -    __init_single_page(pfn_to_page(pfn), pfn, zid, nid, 
>>>> INIT_PAGE_COUNT);
>>>> +    __init_single_page(pfn_to_page(pfn), pfn, zid, nid, 0);
>>>>   }
>>>>   #else
>>>>   static inline void pgdat_set_deferred_range(pg_data_t *pgdat) {}
>>>> @@ -756,8 +756,8 @@ void __meminit 
>>>> reserve_bootmem_region(phys_addr_t start,
>>>>                 init_reserved_page(start_pfn, nid);
>>>>   -            /* Avoid false-positive PageTail() */
>>>> -            INIT_LIST_HEAD(&page->lru);
>>>> +            /* Init page count for reserved region */
>>>
>>> Please add a comment that describes _why_ we initialize the page 
>>> count here.
>>>
>>>> +            init_page_count(page);
>>>>                 /*
>>>>                * no need for atomic set_bit because the struct
>>>> @@ -888,9 +888,17 @@ void __meminit memmap_init_range(unsigned long 
>>>> size, int nid, unsigned long zone
>>>>           }
>>>>             page = pfn_to_page(pfn);
>>>> -        __init_single_page(page, pfn, zone, nid, INIT_PAGE_COUNT);
>>>> -        if (context == MEMINIT_HOTPLUG)
>>>> +
>>>> +        /* If the context is MEMINIT_EARLY, we will init page 
>>>> count and
>>>> +         * mark page reserved in reserve_bootmem_region, the free 
>>>> region
>>>> +         * wouldn't have page count and we will check the pages count
>>>> +         * in __free_pages_core.
>>>> +         */
>>>> +        __init_single_page(page, pfn, zone, nid, 0);
>>>> +        if (context == MEMINIT_HOTPLUG) {
>>>> +            init_page_count(page);
>>>>               __SetPageReserved(page);
>>>
>>> Rather than calling init_page_count() and __SetPageReserved() for
>>> MEMINIT_HOTPLUG you can set flags to INIT_PAGE_COUNT | 
>>> INIT_PAGE_RESERVED
>>> an call __init_single_page() after the check for MEMINIT_HOTPLUG.
>>>
>>> But more generally, I wonder if we have to differentiate HOTPLUG 
>>> here at all.
>>> @David, can you comment please?
>>
>> There are a lot of details to that, and I'll share some I can briefly 
>> think of.
>>
>> 1) __SetPageReserved()
>>
>> I tried removing that a while ago, but there was a blocker (IIRC 
>> something about
>> ZONE_DEVICE). I still have the patches at [1] and I could probably 
>> take a look
>> if that blocker still exists (I recall that something changed at some 
>> point, but
>> I never had the time to follow up).
>>
>> But once we stop setting the pages reserved, we might run into issues 
>> with ...
>>
>>
>> 2) init_page_count()
>>
>> virtio-mem, XEN balloon and HV-balloon add memory blocks that can 
>> contain holes.
>> set_online_page_callback() is used to intercept memory onlining and 
>> to expose
>> only the pages that are not holes to the buddy: calling 
>> generic_online_page() on !hole.
>>
>> Holes are PageReserved but with an initialized page count. Memory 
>> offlining will fail on
>> PageReserved pages -- has_unmovable_pages().
>>
>>
>> At least virtio-mem clears the PageReserved flag of holes when 
>> onlining memory,
>> and currently relies in the page count to be reasonable (so memory 
>> offlining can work).
>>
>> static void virtio_mem_set_fake_offline(unsigned long pfn,
>>                     unsigned long nr_pages, bool onlined)
>> {
>>     page_offline_begin();
>>     for (; nr_pages--; pfn++) {
>>         struct page *page = pfn_to_page(pfn);
>>
>>         __SetPageOffline(page);
>>         if (!onlined) {
>>             SetPageDirty(page);
>>             /* FIXME: remove after cleanups */
>>             ClearPageReserved(page);
>>         }
>>     }
>>     page_offline_end();
>> }
>>
>>
>> For virtio-mem, we could initialize the page count there instead. The 
>> other PV drivers
>> might require a bit more thought.
>>
>>
>> [1] 
>> https://github.com/davidhildenbrand/linux/tree/online_reserved_cleanup
>>
>>>
>>>> +        }
>>>>             /*
>>>>            * Usually, we want to mark the pageblock MIGRATE_MOVABLE,
>>>> 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?
>>>
>>>> -    prefetchw(p);
>>>> -    for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
>>>> -        prefetchw(p + 1);
>>>> +    if (page_count(page)) {
>>>> +        prefetchw(p);
>>>> +        for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
>>>> +            prefetchw(p + 1);
>>>> +            __ClearPageReserved(p);
>>>> +            set_page_count(p, 0);
>>>> +        }
>>>>           __ClearPageReserved(p);
>>>>           set_page_count(p, 0);
>>
>> That looks wrong. if the page count would by pure luck be 0 already 
>> for hotplugged memory,
>> you wouldn't clear the reserved flag.
>>
>> These changes make me a bit nervous.
>
>
> Is 'if (page_count(page) || PageReserved(page))' be safer? Or do I 
> need to do something else?
>

How about the following if statement? But it needs to add more patch 
like v1 ([PATCH 2/4] mm: Introduce MEMINIT_LATE context).

It'll be safer, but more complex. Please comment...

	if (context != MEMINIT_EARLY || (page_count(page) || PageReserved(page)) {