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Date: Mon, 2 Oct 2023 10:30:21 +0200
From: David Hildenbrand <david@...hat.com>
To: Mike Rapoport <rppt@...nel.org>, Yajun Deng <yajun.deng@...ux.dev>
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 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.
--
Cheers,
David / dhildenb
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