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Message-ID: <54bed4d3-631f-7d30-aa2c-f8dd2f2c6804@redhat.com>
Date: Thu, 15 Apr 2021 13:19:59 +0200
From: David Hildenbrand <david@...hat.com>
To: Oscar Salvador <osalvador@...e.de>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: Michal Hocko <mhocko@...nel.org>,
Anshuman Khandual <anshuman.khandual@....com>,
Pavel Tatashin <pasha.tatashin@...een.com>,
Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 4/8] mm,memory_hotplug: Allocate memmap from the added
memory range
On 08.04.21 14:18, Oscar Salvador wrote:
> Physical memory hotadd has to allocate a memmap (struct page array) for
> the newly added memory section. Currently, alloc_pages_node() is used
> for those allocations.
>
> This has some disadvantages:
> a) an existing memory is consumed for that purpose
> (eg: ~2MB per 128MB memory section on x86_64)
> b) if the whole node is movable then we have off-node struct pages
> which has performance drawbacks.
> c) It might be there are no PMD_ALIGNED chunks so memmap array gets
> populated with base pages.
>
> This can be improved when CONFIG_SPARSEMEM_VMEMMAP is enabled.
>
> Vmemap page tables can map arbitrary memory.
> That means that we can simply use the beginning of each memory section and
> map struct pages there.
> struct pages which back the allocated space then just need to be treated
> carefully.
>
> Implementation wise we will reuse vmem_altmap infrastructure to override
> the default allocator used by __populate_section_memmap.
> Part of the implementation also relies on memory_block structure gaining
> a new field which specifies the number of vmemmap_pages at the beginning.
> This patch also introduces the following functions:
>
> - vmemmap_init_space: Initializes vmemmap pages by calling move_pfn_range_to_zone(),
> calls kasan_add_zero_shadow() or the vmemmap range and marks
> online as many sections as vmemmap pages fully span.
> - vmemmap_adjust_pages: Accounts/substract vmemmap_pages to node and zone
> present_pages
> - vmemmap_deinit_space: Undoes what vmemmap_init_space does.
>
This is a bit asynchronous; and the function names are not really expressing what is being done :) I'll try to come up with better names below.
It is worth mentioning that the real "mess" is that we want offline_pages() to properly handle zone->present_pages going to 0. Therefore, we want to manually mess with the present page count.
> Signed-off-by: Oscar Salvador <osalvador@...e.de>
> ---
> drivers/base/memory.c | 64 ++++++++++++++--
> include/linux/memory.h | 8 +-
> include/linux/memory_hotplug.h | 13 ++++
> include/linux/memremap.h | 2 +-
> include/linux/mmzone.h | 7 +-
> mm/Kconfig | 5 ++
> mm/memory_hotplug.c | 162 ++++++++++++++++++++++++++++++++++++++++-
> mm/sparse.c | 2 -
> 8 files changed, 247 insertions(+), 16 deletions(-)
>
> diff --git a/drivers/base/memory.c b/drivers/base/memory.c
> index f209925a5d4e..a5e536a3e9a4 100644
> --- a/drivers/base/memory.c
> +++ b/drivers/base/memory.c
> @@ -173,16 +173,65 @@ static int memory_block_online(struct memory_block *mem)
> {
> unsigned long start_pfn = section_nr_to_pfn(mem->start_section_nr);
> unsigned long nr_pages = PAGES_PER_SECTION * sections_per_block;
> + unsigned long nr_vmemmap_pages = mem->nr_vmemmap_pages;
> + int ret;
> +
> + /*
> + * Although vmemmap pages have a different lifecycle than the pages
> + * they describe (they remain until the memory is unplugged), doing
> + * its initialization and accounting at hot-{online,offline} stage
> + * simplifies things a lot
> + */
I suggest detecting the zone in here and just passing it down to online_pages().
> + if (nr_vmemmap_pages) {
> + ret = vmemmap_init_space(start_pfn, nr_vmemmap_pages, mem->nid,
> + mem->online_type);
> + if (ret)
> + return ret;
> + }
>
> - return online_pages(start_pfn, nr_pages, mem->online_type, mem->nid);
> + ret = online_pages(start_pfn + nr_vmemmap_pages,
> + nr_pages - nr_vmemmap_pages, mem->online_type,
> + mem->nid);
> +
> + /*
> + * Undo the work if online_pages() fails.
> + */
> + if (ret && nr_vmemmap_pages) {
> + vmemmap_adjust_pages(start_pfn, -nr_vmemmap_pages);
> + vmemmap_deinit_space(start_pfn, nr_vmemmap_pages);
> + }
> +
> + return ret;
> }
My take would be doing the present page adjustment after onlining succeeded:
static int memory_block_online(struct memory_block *mem)
{
unsigned long start_pfn = section_nr_to_pfn(mem->start_section_nr);
unsigned long nr_pages = PAGES_PER_SECTION * sections_per_block;
unsigned long nr_vmemmap_pages = mem->nr_vmemmap_pages;
struct zone *zone;
int ret;
zone = mhp_get_target_zone(mem->nid, mem->online_type);
if (nr_vmemmap_pages) {
ret = mhp_init_memmap_on_memory(start_pfn, nr_vmemmap_pages, zone);
if (ret)
return ret;
}
ret = online_pages(start_pfn + nr_vmemmap_pages, nr_pages - nr_vmemmap_pages, zone);
if (ret) {
if (nr_vmemmap_pages)
mhp_deinit_memmap_on_memory(start_pfn, nr_vmemmap_pages);
return ret;
}
/*
* Account once onlining succeeded. If the page was unpopulated,
* it is now already properly populated.
*/
if (nr_vmemmap_pages)
adjust_present_page_count(zone, nr_vmemmap_pages);
return 0;
}
And the opposite:
static int memory_block_offline(struct memory_block *mem)
{
unsigned long start_pfn = section_nr_to_pfn(mem->start_section_nr);
unsigned long nr_pages = PAGES_PER_SECTION * sections_per_block;
unsigned long nr_vmemmap_pages = mem->nr_vmemmap_pages;
struct zone *zone;
int ret;
zone = page_zone(pfn_to_page(start_pfn));
/*
* Unaccount before offlining, such that unpopulated zones can
* properly be torn down in offline_pages().
*/
if (nr_vmemmap_pages)
adjust_present_page_count(zone, -nr_vmemmap_pages);
ret = offline_pages(start_pfn + nr_vmemmap_pages, nr_pages - nr_vmemmap_pages);
if (ret) {
if (nr_vmemmap_pages)
adjust_present_page_count(zone, +nr_vmemmap_pages);
return ret;
}
if (nr_vmemmap_pages)
mhp_deinit_memmap_on_memory(start_pfn, nr_vmemmap_pages);
return 0;
}
Having to do the present page adjustment manually is not completely nice,
but it's easier than manually having to mess with zones becomming populated/unpopulated
outside of online_pages()/offline_pages().
--
Thanks,
David / dhildenb
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