Re: your mail

Vlastimil Babka
2017-04-20T11:56:41Z

On 04/20/2017 10:49 AM, Michal Hocko wrote:
> On Thu 20-04-17 09:28:20, Michal Hocko wrote:
>> On Thu 20-04-17 10:27:55, Joonsoo Kim wrote:
> [...]
>>> Your patch try to add PageReserved() to __pageblock_pfn_to_page(). It
>>> woule make that zone->contiguous usually returns false since memory
>>> used by memblock API is marked as PageReserved() and your patch regard
>>> it as a hole. It invalidates set_zone_contiguous() optimization and I
>>> worry about it.
>>
>> OK, fair enough. I did't consider memblock allocations. I will rethink
>> this patch but there are essentially 3 options
>> - use a different criterion for the offline holes dection. I
>> have just realized we might do it by storing the online
>> information into the mem sections
>> - drop this patch
>> - move the PageReferenced check down the chain into
>> isolate_freepages_block resp. isolate_migratepages_block
>>
>> I would prefer 3 over 2 over 1. I definitely want to make this more
>> robust so 1 is preferable long term but I do not want this to be a
>> roadblock to the rest of the rework. Does that sound acceptable to you?
>
> So I've played with all three options just to see how the outcome would
> look like and it turned out that going with 1 will be easiest in the
> end. What do you think about the following? It should be free of any
> false positives. I have only compile tested it yet.

That looks fine, can't say immediately if fully correct. I think you'll
need to bump SECTION_NID_SHIFT as well and make sure things still fit?
Otherwise looks like nobody needed a new section bit since 2005, so we
should be fine.

> ---
> From 747794c13c0e82b55b793a31cdbe1a84ee1c6920 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.com>
> Date: Thu, 13 Apr 2017 10:28:45 +0200
> Subject: [PATCH] mm: consider zone which is not fully populated to have holes
>
> __pageblock_pfn_to_page has two users currently, set_zone_contiguous
> which checks whether the given zone contains holes and
> pageblock_pfn_to_page which then carefully returns a first valid
> page from the given pfn range for the given zone. This doesn't handle
> zones which are not fully populated though. Memory pageblocks can be
> offlined or might not have been onlined yet. In such a case the zone
> should be considered to have holes otherwise pfn walkers can touch
> and play with offline pages.
>
> Current callers of pageblock_pfn_to_page in compaction seem to work
> properly right now because they only isolate PageBuddy
> (isolate_freepages_block) or PageLRU resp. __PageMovable
> (isolate_migratepages_block) which will be always false for these pages.
> It would be safer to skip these pages altogether, though.
>
> In order to do this patch adds a new memory section state
> (SECTION_IS_ONLINE) which is set in memory_present (during boot
> time) or in online_pages_range during the memory hotplug. Similarly
> offline_mem_sections clears the bit and it is called when the memory
> range is offlined.
>
> pfn_to_online_page helper is then added which check the mem section and
> only returns a page if it is onlined already.
>
> Use the new helper in __pageblock_pfn_to_page and skip the whole page
> block in such a case.
>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
> include/linux/memory_hotplug.h | 21 ++++++++++++++++++++
> include/linux/mmzone.h | 20 ++++++++++++++++++-
> mm/memory_hotplug.c | 3 +++
> mm/page_alloc.c | 5 ++++-
> mm/sparse.c | 45 +++++++++++++++++++++++++++++++++++++++++-
> 5 files changed, 91 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
> index 3c8cf86201c3..fc1c873504eb 100644
> --- a/include/linux/memory_hotplug.h
> +++ b/include/linux/memory_hotplug.h
> @@ -14,6 +14,19 @@ struct memory_block;
> struct resource;
>
> #ifdef CONFIG_MEMORY_HOTPLUG
> +/*
> + * Return page for the valid pfn only if the page is online. All pfn
> + * walkers which rely on the fully initialized page->flags and others
> + * should use this rather than pfn_valid && pfn_to_page
> + */
> +#define pfn_to_online_page(pfn) \
> +({ \
> + struct page *___page = NULL; \
> + \
> + if (online_section_nr(pfn_to_section_nr(pfn))) \
> + ___page = pfn_to_page(pfn); \
> + ___page; \
> +})
>
> /*
> * Types for free bootmem stored in page->lru.next. These have to be in
> @@ -203,6 +216,14 @@ extern void set_zone_contiguous(struct zone *zone);
> extern void clear_zone_contiguous(struct zone *zone);
>
> #else /* ! CONFIG_MEMORY_HOTPLUG */
> +#define pfn_to_online_page(pfn) \
> +({ \
> + struct page *___page = NULL; \
> + if (pfn_valid(pfn)) \
> + ___page = pfn_to_page(pfn); \
> + ___page; \
> + })
> +
> /*
> * Stub functions for when hotplug is off
> */
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 0fc121bbf4ff..cad16ac080f5 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1143,7 +1143,8 @@ extern unsigned long usemap_size(void);
> */
> #define SECTION_MARKED_PRESENT (1UL<<0)
> #define SECTION_HAS_MEM_MAP (1UL<<1)
> -#define SECTION_MAP_LAST_BIT (1UL<<2)
> +#define SECTION_IS_ONLINE (1UL<<2)
> +#define SECTION_MAP_LAST_BIT (1UL<<3)
> #define SECTION_MAP_MASK (~(SECTION_MAP_LAST_BIT-1))
> #define SECTION_NID_SHIFT 2
>
> @@ -1174,6 +1175,23 @@ static inline int valid_section_nr(unsigned long nr)
> return valid_section(__nr_to_section(nr));
> }
>
> +static inline int online_section(struct mem_section *section)
> +{
> + return (section && (section->section_mem_map & SECTION_IS_ONLINE));
> +}
> +
> +static inline int online_section_nr(unsigned long nr)
> +{
> + return online_section(__nr_to_section(nr));
> +}
> +
> +#ifdef CONFIG_MEMORY_HOTPLUG
> +void online_mem_sections(unsigned long start_pfn, unsigned long end_pfn);
> +#ifdef CONFIG_MEMORY_HOTREMOVE
> +void offline_mem_sections(unsigned long start_pfn, unsigned long end_pfn);
> +#endif
> +#endif
> +
> static inline struct mem_section *__pfn_to_section(unsigned long pfn)
> {
> return __nr_to_section(pfn_to_section_nr(pfn));
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index caa58338d121..98f565c279bf 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -929,6 +929,9 @@ static int online_pages_range(unsigned long start_pfn, unsigned long nr_pages,
> unsigned long i;
> unsigned long onlined_pages = *(unsigned long *)arg;
> struct page *page;
> +
> + online_mem_sections(start_pfn, start_pfn + nr_pages);
> +
> if (PageReserved(pfn_to_page(start_pfn)))
> for (i = 0; i < nr_pages; i++) {
> page = pfn_to_page(start_pfn + i);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 5d72d29a6ece..fa752de84eef 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1353,7 +1353,9 @@ struct page *__pageblock_pfn_to_page(unsigned long start_pfn,
> if (!pfn_valid(start_pfn) || !pfn_valid(end_pfn))
> return NULL;
>
> - start_page = pfn_to_page(start_pfn);
> + start_page = pfn_to_online_page(start_pfn);
> + if (!start_page)
> + return NULL;
>
> if (page_zone(start_page) != zone)
> return NULL;
> @@ -7686,6 +7688,7 @@ __offline_isolated_pages(unsigned long start_pfn, unsigned long end_pfn)
> break;
> if (pfn == end_pfn)
> return;
> + offline_mem_sections(pfn, end_pfn);
> zone = page_zone(pfn_to_page(pfn));
> spin_lock_irqsave(&zone->lock, flags);
> pfn = start_pfn;
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 6903c8fc3085..79017f90d8fc 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -185,7 +185,8 @@ void __init memory_present(int nid, unsigned long start, unsigned long end)
> ms = __nr_to_section(section);
> if (!ms->section_mem_map)
> ms->section_mem_map = sparse_encode_early_nid(nid) |
> - SECTION_MARKED_PRESENT;
> + SECTION_MARKED_PRESENT |
> + SECTION_IS_ONLINE;
> }
> }
>
> @@ -590,6 +591,48 @@ void __init sparse_init(void)
> }
>
> #ifdef CONFIG_MEMORY_HOTPLUG
> +
> +/* Mark all memory sections within the pfn range as online */
> +void online_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
> +{
> + unsigned long pfn;
> +
> + for (pfn = start_pfn; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
> + unsigned long section_nr = pfn_to_section_nr(start_pfn);
> + struct mem_section *ms;
> +
> + /* onlining code should never touch invalid ranges */
> + if (WARN_ON(!valid_section_nr(section_nr)))
> + continue;
> +
> + ms = __nr_to_section(section_nr);
> + ms->section_mem_map |= SECTION_IS_ONLINE;
> + }
> +}
> +
> +#ifdef CONFIG_MEMORY_HOTREMOVE
> +/* Mark all memory sections within the pfn range as online */
> +void offline_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
> +{
> + unsigned long pfn;
> +
> + for (pfn = start_pfn; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
> + unsigned long section_nr = pfn_to_section_nr(start_pfn);
> + struct mem_section *ms;
> +
> + /*
> + * TODO this needs some double checking. Offlining code makes
> + * sure to check pfn_valid but those checks might be just bogus
> + */
> + if (WARN_ON(!valid_section_nr(section_nr)))
> + continue;
> +
> + ms = __nr_to_section(section_nr);
> + ms->section_mem_map &= ~SECTION_IS_ONLINE;
> + }
> +}
> +#endif
> +
> #ifdef CONFIG_SPARSEMEM_VMEMMAP
> static inline struct page *kmalloc_section_memmap(unsigned long pnum, int nid)
> {
>