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

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

On 2023/10/2 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?