Re: [PATCH] mm: Teach pfn_to_online_page() about ZONE_DEVICE section collisions

[David Hildenbrand <david@redhat.com>]

> Am 06.01.2021 um 05:08 schrieb Dan Williams <dan.j.williams@intel.com>:
> 
> While pfn_to_online_page() is able to determine pfn_valid() at
> subsection granularity it is not able to reliably determine if a given
> pfn is also online if the section is mixed with ZONE_DEVICE pfns.
> 
> Update move_pfn_range_to_zone() to flag (SECTION_TAINT_ZONE_DEVICE) a
> section that mixes ZONE_DEVICE pfns with other online pfns. With
> SECTION_TAINT_ZONE_DEVICE to delineate, pfn_to_online_page() can fall
> back to a slow-path check for ZONE_DEVICE pfns in an online section.
> 
> With this implementation of pfn_to_online_page() pfn-walkers mostly only
> need to check section metadata to determine pfn validity. In the
> rare case of mixed-zone sections the pfn-walker will skip offline
> ZONE_DEVICE pfns as expected.
> 
> Other notes:
> 
> Because the collision case is rare, and for simplicity, the
> SECTION_TAINT_ZONE_DEVICE flag is never cleared once set.
> 
> pfn_to_online_page() was already borderline too large to be a macro /
> inline function, but the additional logic certainly pushed it over that
> threshold, and so it is moved to an out-of-line helper.
> 
> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Reported-by: Michal Hocko <mhocko@suse.com>
> Reported-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
> 
> This compiles and passes the nvdimm unit tests, but I have not tested
> with pfn walkers in the presence of ZONE_DEVICE collisions.
> 
> 
> include/linux/memory_hotplug.h |   17 +---------
> include/linux/mmzone.h         |   22 ++++++++----
> mm/memory_hotplug.c            |   71 ++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 87 insertions(+), 23 deletions(-)
> 
> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
> index 15acce5ab106..3d99de0db2dd 100644
> --- a/include/linux/memory_hotplug.h
> +++ b/include/linux/memory_hotplug.h
> @@ -16,22 +16,7 @@ struct resource;
> struct vmem_altmap;
> 
> #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;                   \
> -    unsigned long ___pfn = pfn;                   \
> -    unsigned long ___nr = pfn_to_section_nr(___pfn);       \
> -                                   \
> -    if (___nr < NR_MEM_SECTIONS && online_section_nr(___nr) && \
> -        pfn_valid_within(___pfn))                   \
> -        ___page = pfn_to_page(___pfn);               \
> -    ___page;                           \
> -})
> +struct page *pfn_to_online_page(unsigned long pfn);
> 
> /*
>  * Types for free bootmem stored in page->lru.next. These have to be in
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index b593316bff3d..0b5c44f730b4 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1273,13 +1273,14 @@ extern size_t mem_section_usage_size(void);
>  *      which results in PFN_SECTION_SHIFT equal 6.
>  * To sum it up, at least 6 bits are available.
>  */
> -#define    SECTION_MARKED_PRESENT    (1UL<<0)
> -#define SECTION_HAS_MEM_MAP    (1UL<<1)
> -#define SECTION_IS_ONLINE    (1UL<<2)
> -#define SECTION_IS_EARLY    (1UL<<3)
> -#define SECTION_MAP_LAST_BIT    (1UL<<4)
> -#define SECTION_MAP_MASK    (~(SECTION_MAP_LAST_BIT-1))
> -#define SECTION_NID_SHIFT    3
> +#define SECTION_MARKED_PRESENT        (1UL<<0)
> +#define SECTION_HAS_MEM_MAP        (1UL<<1)
> +#define SECTION_IS_ONLINE        (1UL<<2)
> +#define SECTION_IS_EARLY        (1UL<<3)
> +#define SECTION_TAINT_ZONE_DEVICE    (1UL<<4)
> +#define SECTION_MAP_LAST_BIT        (1UL<<5)
> +#define SECTION_MAP_MASK        (~(SECTION_MAP_LAST_BIT-1))
> +#define SECTION_NID_SHIFT        3
> 
> static inline struct page *__section_mem_map_addr(struct mem_section *section)
> {
> @@ -1318,6 +1319,13 @@ static inline int online_section(struct mem_section *section)
>    return (section && (section->section_mem_map & SECTION_IS_ONLINE));
> }
> 
> +static inline int online_device_section(struct mem_section *section)
> +{
> +    unsigned long flags = SECTION_IS_ONLINE | SECTION_TAINT_ZONE_DEVICE;
> +
> +    return section && ((section->section_mem_map & flags) == flags);
> +}
> +
> static inline int online_section_nr(unsigned long nr)
> {
>    return online_section(__nr_to_section(nr));
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index f9d57b9be8c7..9f36968e6188 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -300,6 +300,47 @@ static int check_hotplug_memory_addressable(unsigned long pfn,
>    return 0;
> }
> 
> +/*
> + * 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
> + */
> +struct page *pfn_to_online_page(unsigned long pfn)
> +{
> +    unsigned long nr = pfn_to_section_nr(pfn);
> +    struct dev_pagemap *pgmap;
> +    struct mem_section *ms;
> +
> +    if (nr >= NR_MEM_SECTIONS)
> +        return NULL;
> +
> +    ms = __nr_to_section(nr);
> +
> +    if (!online_section(ms))
> +        return NULL;
> +
> +    if (!pfn_valid_within(pfn))
> +        return NULL;
> +
> +    if (!online_device_section(ms))
> +        return pfn_to_page(pfn);
> +
> +    /*
> +     * Slowpath: when ZONE_DEVICE collides with
> +     * ZONE_{NORMAL,MOVABLE} within the same section some pfns in
> +     * the section may be 'offline' but 'valid'. Only
> +     * get_dev_pagemap() can determine sub-section online status.
> +     */
> +    pgmap = get_dev_pagemap(pfn, NULL);
> +    put_dev_pagemap(pgmap);
> +
> +    /* The presence of a pgmap indicates ZONE_DEVICE offline pfn */
> +    if (pgmap)
> +        return NULL;
> +    return pfn_to_page(pfn);
> +}
> +EXPORT_SYMBOL_GPL(pfn_to_online_page);
> +
> /*
>  * Reasonably generic function for adding memory.  It is
>  * expected that archs that support memory hotplug will
> @@ -678,6 +719,27 @@ static void __meminit resize_pgdat_range(struct pglist_data *pgdat, unsigned lon
>    pgdat->node_spanned_pages = max(start_pfn + nr_pages, old_end_pfn) - pgdat->node_start_pfn;
> 
> }
> +
> +static int zone_id(const struct zone *zone)
> +{
> +    struct pglist_data *pgdat = zone->zone_pgdat;
> +
> +    return zone - pgdat->node_zones;
> +}
> +
> +static void section_taint_zone_device(struct zone *zone, unsigned long pfn)
> +{
> +    struct mem_section *ms = __nr_to_section(pfn_to_section_nr(pfn));
> +
> +    if (zone_id(zone) != ZONE_DEVICE)
> +        return;
> +
> +    if (IS_ALIGNED(pfn, PAGES_PER_SECTION))
> +        return;

I think you can simplify this. Just taint any early section here, independent of the pfn.