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Date: Sun, 8 Oct 2023 16:57:22 +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 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@...ux.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?
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